[RTC-337] Add FileEndpoint

[Rados13]

https://membraneframework.atlassian.net/browse/RTC-337?atlOrigin=eyJpIjoiZjRkNTEzMDliNmEwNDhkNDk5YWIxNzhkMzhmODQ2MWQiLCJwIjoiaiJ9

[codecov[bot]]

Codecov Report

Merging #323 (d8f22f2) into master (a19b337) will increase coverage by 1.17%. The diff coverage is 78.78%.

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
+ Coverage   52.67%   53.85%   +1.17%     
==========================================
  Files          38       41       +3     
  Lines        1978     2061      +83     
==========================================
+ Hits         1042     1110      +68     
- Misses        936      951      +15     

Files	Coverage Δ
engine/lib/membrane_rtc_engine/engine.ex	82.17% <ø> (ø)
engine/lib/membrane_rtc_engine/track.ex	33.33% <ø> (ø)
file/lib/file_source_endpoint.ex	87.87% <100.00%> (ø)
hls/lib/hls_endpoint.ex	50.42% <ø> (ø)
integration_test/test/support/hls_storage.ex	91.66% <ø> (ø)
file/test/support/file_sink_endpoint.ex	73.07% <73.07%> (ø)

... and 3 files with indirect coverage changes

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[sgfn]

One more thing: we should edit /README.md, /engine/README.md (add mix test.file), and the moduledoc in /engine/lib/membrane_rtc_engine/engine.ex (section about Endpoints)

[mickel8]

And as we discussed, let's include H264 and Opus parsers inside File endpoint