[P2P] Modernize STUN and P2P code to use std::span

This change replaces (const void*, size_t) and (const char*, size_t)
patterns with std::span<const uint8_t> across the STUN and P2P codebases.

Key changes:
- Modernized api/transport/stun.h (StunMessage, StunByteStringAttribute).
- Modernized p2p/base/port_interface.h, port.h and subclasses
  (UDPPort, TCPPort, TurnPort, TestPort).
- Modernized p2p/base/connection.h and stun_request.h.
- Added [[deprecated]] shims for all public API changes to maintain
  backward compatibility.
- Updated all internal callers and unit tests.
- Fixed a hardening assertion crash in api/transport/stun_unittest.cc.

Bug: webrtc:42225170
Change-Id: I3ef7b21996b0c17a6292bce75be983112c9e6649
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/466420
Reviewed-by: Evan Shrubsole <eshr@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47572}