Move Sframe mode from encryptor to SframeEncryptionConfig

Sframe mode determines how the send pipeline applies encryption, not how
the encryptor performs cryptographic operations. Introduce
SframeEncryptionConfig to carry the mode and encryptor together through
the send pipeline.

Keep mode in SframeEncryptorInit as the application-facing input, but
construct SframeEncryptor using only the cipher suite. Remove its stored
mode and the mode() accessor from the media encryptor interface.

Store an optional SframeEncryptionConfig in RtpSenderBase and
SframeSendOptions. Rename the media-channel setter to
SetSframeEncryptionConfig and propagate the configuration during
installation, SSRC binding, and video send-stream creation or
recreation. RtpVideoSender forwards it to each RTPSenderVideo, including
simulcast streams.

Keep the requirement for Sframe protection independent of the encryption
configuration. Rename the RTP-level flag to
sframe_required_by_negotiation to clarify that it reflects applied SDP.
Preserve the existing frame-dropping behavior while Sframe encryption is
not yet wired into the RTP sender; audio propagation remains a TODO.

Bug: webrtc:479862368
Change-Id: I7e0ffbf1843dd0231354460a25ee052f26552ae1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/507741
Commit-Queue: Kacper Wasniowski <kwasniow@cisco.com>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48819}