Replace EncoderSwitchRequestCallback with absl::AnyInvocable

This refactoring replaces the EncoderSwitchRequestCallback interface
with absl::AnyInvocable in VideoStreamEncoderSettings.

To ensure the callback survives stream recreation, the callback binding
has been moved to RecreateWebRtcStream, relying on a new `send_channel_`
back-pointer within WebRtcVideoSendStream.

Bug: b/478050997
Change-Id: I1f587d563553c4f2f3d8fd02c4d9ed4473829dc4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/454480
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47087}