feat: add the missing peer connection signaling and control calls

[devopvoid]

Closes more of the gap with the Android SDK's PeerConnection, and fixes two bugs found along the way.

Fixes

• Data channel priority was ignored. RTCDataChannelInit.priority was looked up natively but never copied, so every channel was created, and announced to the remote peer, as low priority. It is now applied, and RTCDataChannel.getPriority() reads it back (a priority the remote peer announces between two levels rounds up to the next). WebRTC only uses it to share the SCTP transport when the WebRTC-DataChannelMessageInterleaving field trial is on for both peers; the javadoc and guide say so.
• A malformed certificate aborted the JVM. A certificate in RTCConfiguration.certificates that did not parse hit an RTC_CHECK. createPeerConnection and setConfiguration now throw a RuntimeException (a NullPointerException for a null entry), and catch conversion errors instead of letting a C++ exception leave the JNI call.

New API

RTCPeerConnection:

• addIceCandidate(candidate, AddIceCandidateObserver): queued behind pending offer/answer operations, as the standard requires, and reports whether the candidate was accepted. The existing addIceCandidate(candidate) fails silently.
• removeIceCandidate(candidate): returns whether the candidate was found and removed.
• setLocalDescription(observer): creates the offer or answer itself and applies it, as perfect negotiation needs.
• setBitrate(min, current, max): limits the connection's send bandwidth, in bps; null leaves a limit unchanged.
• setAudioPlayout(boolean), setAudioRecording(boolean): act on the factory's audio device module, so they apply to all of its connections. Tracks fed by a CustomAudioSource are not affected by recording being off.

PeerConnectionObserver:

• onIceCandidateRemoved(RTCIceCandidate): a local candidate went away.
• onSelectedCandidatePairChanged(RTCCandidatePairChangeEvent): both candidates of the selected pair, the reason for the switch, the last time data was received and the estimated disconnection time. Its default implementation passes the remote address, port and type on to the existing three-argument callback, so observers that override that one keep working.

Not added: getCertificate(), since getConfiguration().certificates already returns the configured certificate, which is all Android's version returns.

Exception handling

The new callbacks hand a Java exception thrown by the application to the thread's uncaught exception handler, instead of throwing it as a C++ exception through WebRTC's frames (which leaks WebRTC's locks). ReportUncaughtException moved from the data channel send observer to WebRTCUtils to be shared. The older PeerConnectionObserver callbacks still use ExceptionCheck and are left for a separate change.

Docs

• get-started.md: implicit setLocalDescription, addIceCandidate with an observer, candidate removal and the pair change event.
• peer-connection-config.md: a section on setBitrate and the audio switches, and how certificates are handled.
• data-channels.md: what priority does.
• directionality.md: the sample passed a null observer to setLocalDescription, which throws; it now uses the implicit form.

Tests

• RTCPeerConnectionSignalingTests (new, two peers): implicit offer and answer, candidates added with an observer and then removed, a candidate for an unknown MID rejected, the full pair change event, the older pair callback still firing, and a remote channel getting the announced priority.
• RTCPeerConnectionTests: setBitrate validation, audio switches, addIceCandidate without a remote description, null arguments, implicit offer on one peer.
• RTCDataChannelTests.priority, RTCConfigurationFieldsTests.malformedCertificateThrows and setConfigurationRejectsMalformedCertificate.

The full webrtc suite passes on Windows (249 run, 0 failures, 2 skipped), and the affected classes pass under -Pjni-check with no JNI errors. onIceCandidateRemoved has no test, since a local candidate is only removed on a network change.