Add DTLS wire format framing stub to RtcTransport

Introduce PacketProcessor, a protocol-agnostic hook that a
DatagramConnection calls on every outgoing and incoming packet, and
DtlsPacketProcessor, its DTLS-only implementation for the new
WireProtocol::kDtlsWithFeedback mode.

DtlsPacketProcessor frames app payloads with a 1-byte type header and a
16-bit transport sequence number, supporting the three wire formats:
data (0x01), reserved values (0x00, 0x02-0xFF) are dropped with a warning; 0x02 and 0x03 are set aside for the congestion-control feedback wire format, which is not yet defined (its format is still under discussion) and will be added in a follow-up along with the congestion controller hookup.

The connection stays protocol-agnostic: it frames via
ProcessOutgoingPacket() and de-frames via ProcessIncomingPacket(),
delivering the stripped payload through the common receive path.

Bug: chromium:443019066
Change-Id: Icc0f74e5bb31ef1fac1419ec2859e3c26af06555
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/489220
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48488}