webrtc: Add support for unordered unreliable data channels

[PaulOlteanu]

Fixes #371. Most of the change is in RTCDataChannel::open to match https://github.com/pion/webrtc/blob/4ef00e6e5f78647305cdd08568d16f49ae6190eb/datachannel.go#L130-L152

[codecov]

Codecov Report

Attention: Patch coverage is 62.16216% with 28 lines in your changes missing coverage. Please review.

Project coverage is 60.61%. Comparing base (23bbc1f) to head (ebbf745).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
webrtc/src/data_channel/data_channel_test.rs	61.11%	2 Missing and 26 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #609      +/-   ##
==========================================
+ Coverage   60.59%   60.61%   +0.02%     
==========================================
  Files         482      471      -11     
  Lines       47981    48239     +258     
  Branches    12478    12537      +59     
==========================================
+ Hits        29075    29242     +167     
- Misses       9665     9674       +9     
- Partials     9241     9323      +82     

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[rainliu]

change this line to

_ => {

otherwise, compile error due to missing one case handling

[PaulOlteanu]

All the cases are covered (and I compiled and ran tests locally fine). The CI failures don't seem related to my changes