Wire Sframe enable callback through sender and receiver constructors.

This CL implements the C++ plumbing for the SFrame enable signal
described in the WebRTC Encoded Transform spec
(https://w3c.github.io/webrtc-encoded-transform/#attribute, Section
2.2).

Per the spec, when an SFrame transform is set on an RTCRtpSender or
RTCRtpReceiver, the setter must propagate [[useSFrame]] = true to the
owning RTCRtpTransceiver and trigger SDP renegotiation. If the
transceiver's [[useSFrame]] has already been negotiated to false, it
must throw InvalidModificationError.

This is modeled by wiring an on_sframe_enable_requested callback from
each sender/receiver to RtpTransceiver::TryToEnableSframe(). When the
application calls CreateSframeEncrypterOrError() (sender) or
CreateSframeDecrypterOrError() (receiver), the callback fires, which
sets sframe_enabled_ = true on the transceiver and triggers
on_negotiation_needed_(). After negotiation completes,
ApplySframeEnabled() locks the state — matching the spec's behavior
where [[useSFrame]] becomes immutable once associated with a media
description.

Bug: webrtc:479862368
Change-Id: Ie5edd365aa4896bfda44cf34a2ab52dfe32bb441
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/459460
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47536}