Improve pluggability and testability across pion modules though Interfaces

Summary

This is a pion/webrtc v4 item. We'd like to use interfaces between pion modules as much as possible to provide users with true pluggability. To achieve this, we'd also need to introduce customizable factories.

Motivation

Pion WebRTC is comprised of many modules (a poly-repo solution) with pluggability in mind. This has allowed us to manage each WebRTC sub-components with independent versions. It is, however, not flexible enough to achieve intended pluggability because a module depends on another via direct struct pointers rather than via interfaces.

By converting struct pointer to an interface, we can achieve the following:

Testing a module much easier by mocking the dependencies
It allows us to dynamically swap multiple versions of a module on the fly (e.g. A/B testing in benching marking and/or in production environment)
It allows us to use custom implementation of a module along with stock pion modules.

Proposal: New Interfaces and CustomFactories

classDiagram
    `webrtc.API` --> `webrtc.CustomFactories` : newXxx()
    `webrtc.API` --> `webrtc.CustomFactories` : Replace factory
    `webrtc.CustomFactories` --> `ice.Agent` : &lt&ltcreate&gt&gt
    `ice.Agent` --> `ice.Conn` : &lt&ltcreate&gt&gt
    `webrtc.CustomFactories` --> `dtls.StreamRTP` : &lt&ltcreate&gt&gt
    `dtls.StreamRTP` --> `dtls.ReadStreamRTP` : &lt&ltcreate&gt&gt
    `dtls.StreamRTP` --> `dtls.WriteStreamRTP` : &lt&ltcreate&gt&gt
    `dtls.StreamRTCP` --> `dtls.ReadStreamRTCP` : &lt&ltcreate&gt&gt
    `dtls.StreamRTCP` --> `dtls.WriteStreamRTCP` : &lt&ltcreate&gt&gt
    `webrtc.CustomFactories` --> `dtls.StreamRTCP` : &lt&ltcreate&gt&gt
    `webrtc.CustomFactories` --> `sctp.Association` : &lt&ltcreate&gt&gt
    `sctp.Association` --> `sctp.Stream` : &lt&ltcreate&gt&gt
    `webrtc.CustomFactories` --> `datachannel.DataChannel` : &lt&ltcreate&gt&gt
    <<interface>>`ice.Agent`
    <<interface>>`ice.Conn`
    <<interface>>`dtls.StreamRTP`
    <<interface>>`dtls.ReadStreamRTP`
    <<interface>>`dtls.WriteStreamRTP`
    <<interface>>`dtls.StreamRTCP`
    <<interface>>`dtls.ReadStreamRTCP`
    <<interface>>`dtls.WriteStreamRTCP`
    <<interface>>`sctp.Association`
    <<interface>>`sctp.Stream`
    <<interface>>`datachannel.DataChannel`

Plan (high level)

Use this issue as an umbrella for individual issues/pull-requests in different modules related to this work. Detailed plan will follow.
Create a quick migration guide (e.g. *sctp.Assocation => sctp.Association)

pion / webrtc