Hide VP8 and Opus Payloader/Depayloader modules

[mickel8]

Sorry, I don't want to mess in your PR but I have a couple of observations:

• depayloaders does not take any options right now so we can get rid of them
• all payloaders can take the same argument - max_payload_size
• if the two above are true, we can hide everything except ExWebRTC.RTP.Payloader and ExWebRTC.RTP.Depayloader - user doesn't have to know about specific payloader/depayloader modules
• we can extract payloader/depayloader behaviours into separate modules so they are private too. If we want to provide automatic dispatching, user won't be able to implement those behaviours as our dispatcher's new function has to be modified anyway

An alternative approach would be to use Protocols. The API would look like this:

defprotocol ExWebRTC.RTP.Payloader do
  def payload(t(), data) 
end

defprotocol ExWebRTC.RTP.Depayloader do
  def depayload(t(), data)
end

defmodule ExWebRTC.RTP.Utils do
  alias ExWebRTC.RTPCodecParameters

  @spec new_payloader(RTPCodecParameters.t()) :: ExWebRTC.RTP.Payloader.t()
  def new_payloader(codec_params) do
  end

  @spec new_depayloader(RTPCodecParameters.t()) :: ExWebRTC.RTP.Depayloader.t()
  def new_depayloader(codec_params) do
  end
end

This way, user can use RTP utils to automatically find payloader/depayloader or implement their own payloade/depayloader and still use our protocols. This would be beneficial if we used those protocols internally, in PeerConnection but we don't right now.

Any thoughts?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.52%. Comparing base (b142d04) to head (be6c244).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## sgfn/payloader-behaviour #154 +/- ## ========================================================= Coverage 88.51% 88.52% ========================================================= Files 38 38 Lines 1907 1908 +1 ========================================================= + Hits 1688 1689 +1 Misses 219 219 ``` | [Files](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/154?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc) | Coverage Δ | | |---|---|---| | [lib/ex\_webrtc/rtp/depayloader.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/154?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp%2Fdepayloader.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHAvZGVwYXlsb2FkZXIuZXg=) | `100.00% <100.00%> (ø)` | | | [lib/ex\_webrtc/rtp/opus/depayloader.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/154?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp%2Fopus%2Fdepayloader.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHAvb3B1cy9kZXBheWxvYWRlci5leA==) | `100.00% <ø> (ø)` | | | [lib/ex\_webrtc/rtp/opus/payloader.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/154?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp%2Fopus%2Fpayloader.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHAvb3B1cy9wYXlsb2FkZXIuZXg=) | `100.00% <ø> (ø)` | | | [lib/ex\_webrtc/rtp/payloader.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/154?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp%2Fpayloader.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHAvcGF5bG9hZGVyLmV4) | `100.00% <100.00%> (ø)` | | | [lib/ex\_webrtc/rtp/vp8/depayloader.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/154?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp%2Fvp8%2Fdepayloader.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHAvdnA4L2RlcGF5bG9hZGVyLmV4) | `77.27% <100.00%> (ø)` | | | [lib/ex\_webrtc/rtp/vp8/payload.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/154?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp%2Fvp8%2Fpayload.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHAvdnA4L3BheWxvYWQuZXg=) | `90.00% <ø> (ø)` | | | [lib/ex\_webrtc/rtp/vp8/payloader.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/154?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp%2Fvp8%2Fpayloader.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHAvdnA4L3BheWxvYWRlci5leA==) | `100.00% <ø> (ø)` | | ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/154?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/154?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc). Last update [b142d04...be6c244](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/154?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc).