feat: map the remaining sender encoding parameters

[devopvoid]

RTCRtpEncodingParameters and RTCRtpSendParameters gain the encoding parameters WebRTC has and the Java API didn't. This makes simulcast, SVC and degradation behavior controllable from Java.

API

On RTCRtpEncodingParameters:

Field	What it does
rid	RTP stream ID, for simulcast through RTCRtpTransceiverInit.sendEncodings
scaleResolutionDownTo	Absolute size cap (new RTCResolutionRestriction); takes precedence over scaleResolutionDownBy
scalabilityMode	SVC mode, e.g. L1T3, L3T3_KEY
numTemporalLayers	The older way to ask for temporal layers
bitratePriority, networkPriority	Sender priorities (share of bitrate, DSCP)
adaptivePtime	Longer audio packets at low bitrates
codec	A codec of its own for the encoding, e.g. mixed-codec simulcast

On RTCRtpSendParameters: degradationPreference, with the new RTCDegradationPreference enum.

On RTCRtpCodecCapability: getScalabilityModes(), the modes a codec supports, i.e. what scalabilityMode accepts.

Behavior, checked against WebRTC's source

• Priorities: bitratePriority and networkPriority apply to the whole sender. WebRTC accepts them on the first encoding only (PerSenderRtpEncodingParameterHasValue), and the Javadoc says so.
• rid: can't be changed after the transceiver is created; setParameters() throws.
• Scalability modes: an unsupported one makes setParameters() throw (CheckScalabilityModeValues).
• Degradation default: unset, WebRTC keeps the resolution of screen shares and of detail/text content hints, and the frame rate of other video. BALANCED is only a default behind a field trial.

Testing

RTCRtpEncodingParametersTests (9 tests):

• simulcast rids, scales and priorities round-trip through addTransceiver
• a changed rid is rejected
• priorities on a later encoding are rejected
• scaleResolutionDownTo and degradationPreference round-trip
• adaptivePtime round-trips on audio
• in a live VP8 call, L1T3 is applied and frames keep flowing
• L3T3 is rejected for VP8, and VP8's capability lists L1T3 but not L3T3
• a per-encoding codec round-trips

mvn -pl webrtc test: 193 tests pass. -Pjni-check: 193 tests pass, with no FATAL ERROR in native method.

Docs

The media constraints guide (docs/guide/media/constraints.md) has new sections on absolute resolution caps, degradation preference, simulcast, scalability modes and priorities.