Populate RTPTransceiver stopped flag

[boks1971]

There is use of t.stopped in peerconnection.go, but that flag is not set/used in RTPTransceiver at all. This PR sets that when a transceiver is stopped and provides an API to check if a transceiver is stopped.

@Sean-Der @davidzhao @cnderrauber The fact that code does not seem to get tripped makes me think I am missing something, but I noticed the ☝️ (i. e. stopped flag in RTPTransceiver was not set/used) and made this PR to address this. Have tested this change with LiveKit and it seems fine.

[codecov]

Codecov Report

Base: 77.52% // Head: 77.59% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (097d94f) compared to base (42dc0d4).
Patch coverage: 91.66% of modified lines in pull request are covered.

❗ Current head 097d94f differs from pull request most recent head 71f4e4b. Consider uploading reports for the commit 71f4e4b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2336      +/-   ##
==========================================
+ Coverage   77.52%   77.59%   +0.07%     
==========================================
  Files          87       87              
  Lines        9262     9266       +4     
==========================================
+ Hits         7180     7190      +10     
+ Misses       1648     1644       -4     
+ Partials      434      432       -2     

Flag	Coverage Δ
go	79.37% <91.66%> (+0.08%)	⬆️
wasm	70.16% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
peerconnection.go	78.23% <83.33%> (+0.16%)	⬆️
rtptransceiver.go	87.71% <100.00%> (+0.36%)	⬆️
icetransport.go	68.42% <0.00%> (-1.22%)	⬇️
dtlstransport.go	64.90% <0.00%> (+1.86%)	⬆️

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[cnderrauber]

Pion's transceiver don't have a real stopped state now, it call stop method when state go to inactive, so in pion, a transceiver after stop (actually it not set stopped flag, just transit to inactive) can be reused. #2226 also mentioned this. Actually, https://datatracker.ietf.org/doc/html/rfc8829#section-4.2.1 said that a stopped can't be reused, so this PR would comply with the rfc, but changed transceiver reuse behavior, might cause problem if application relied on it.
There are #1943 and #1969 do similar things and revert it, don't know why, @Sean-Der might have suggestion about this.

[boks1971]

Thank you @cnderrauber . Did not realise this path had already been done. Maybe, we should just remove the stopped flag to avoid confusion.