Add SFrame packet buffer for RTP-level frame assembly

This CL adds the foundational components for SFrame receive-side support
in WebRTC, as specified in draft-ietf-avtcore-rtp-sframe. These
components sit upstream of the existing PacketBuffer and handle
SFrame-specific parsing and frame boundary validation before packets
enter the standard video receive pipeline.

Three new components, with BUILD.gn targets and unit tests:

SFrameDescriptor -- header-only struct representing the 1-byte SFrame
  payload descriptor (S/E/T bits: start-of-frame, end-of-frame,
  per-packet vs per-frame mode).

SframeRtpPacketReceived -- lightweight wrapper pairing an
  RtpPacketReceived with its parsed SFrameDescriptor, so downstream code
  can inspect S/E/T bits without re-parsing.

SFramePacketBuffer -- circular buffer that collects SFrame RTP packets
  and validates complete S->E runs (contiguous start-to-end sequences)
  before releasing them for decryption and depacketization.  Follows the
  same circular-buffer pattern as the existing PacketBuffer: starts at 96
  slots, doubles up to 2048, validates frame consistency (T-bit, payload
  type, timestamp), and supports ClearTo for window management.

Why this is needed

SFrame (draft-ietf-avtcore-rtp-sframe) adds a 1-byte payload descriptor
to each RTP packet indicating frame boundaries and encryption mode.
Before SFrame packets can enter the existing codec depacketizer
pipeline, we need to:

1. Parse and strip the SFrame descriptor byte
  2. Collect packets into complete frames (S->E runs) at the RTP level
  3. Branch on the T-bit to determine whether decryption is per-packet
     (T=1) or per-frame (T=0)

The existing PacketBuffer operates on codec-depacketized payloads and
cannot perform this pre-processing. A separate buffer layer is required.

More on architecture plan:
https://github.com/webex/webrtc-sframe/tree/sframe-receive-pipeline

Bug: webrtc:479862368
Change-Id: I40c076739a7e237eae344a6a3282948c53d5420b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/466920
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47856}