Add RtpDepacketizerSframe (SFrame RTP depacketizer)

Introduces a stateless parser for the 1-byte SFrame RTP payload
descriptor. RtpDepacketizerSframe::Parse() takes an RtpPacketReceived,
parses the S/E/T bits into an SFrameDescriptor, strips the descriptor
byte from the payload, and returns the result as an
SframeRtpPacketReceived which downstream code (the SFrame packet
buffer) can use for assembly without re-parsing the wire format.

Bug: webrtc:479862368
Change-Id: I9fc2916cd84ff1fc798c28ee6ed5739066a37491
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/480120
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48067}