Reland "RtpSender: Create the Sframe encryptor and hand out a worker proxy"

This reverts commit 348e5ff7247ceffe58d8fa3daf8fb917d74f68c9.

Reason for revert: Sframe library updated to support std::span

Original change's description:
> Revert "RtpSender: Create the Sframe encryptor and hand out a worker proxy"
>
> This reverts commit 8354b7c19d32a3dbe7268319af6085e6ec9ec099.
>
> Reason for revert: breaks webrtc-internal bot. Compile error in sframe
>
> Failure Link: https://fusion2.corp.google.com/invocations/e9a2a344-3234-4d69-a6e9-b95f8700b396/targets/%2F%2Fthird_party%2Fwebrtc:webrtc_all_cc/log?q=label:session_id%3Dbtbs-224212c0-8627-4ddc-9137-60e85c0d9b6c
>
> Original change's description:
> > RtpSender: Create the Sframe encryptor and hand out a worker proxy
> >
> > Implements `RtpSenderBase::CreateSframeEncryptorOrError`
> >
> > When called, it creates a `SframeEncryptor` on the worker thread and —
> > if an SSRC is already assigned — immediately pushes it to the media
> > channel via `SetSframeEncryptor`. If the SSRC is not yet known,
> > `SetSsrc` is updated to push the encryptor to the channel once an SSRC
> > is assigned.
> >
> > The returned `SframeEncryptorInterface` is wrapped in a new
> > `SframeEncryptorProxy` that marshals `SetEncryptionKey` calls to the
> > worker thread, so callers can safely invoke it from any other thread.
> >
> > Bug: webrtc:479862368
> > Change-Id: Ic647c4658fa672c820d09a13c34ee0b158f3ae89
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/493780
> > Commit-Queue: Björn Terelius <terelius@webrtc.org>
> > Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
> > Reviewed-by: Björn Terelius <terelius@webrtc.org>
> > Cr-Commit-Position: refs/heads/main@{#48542}
>
> Bug: webrtc:479862368
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Change-Id: I7e24587bf67f8f4a5f85a29c672fbccc22a566b8
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/501520
> Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
> Owners-Override: Ilya Nikolaevskiy <ilnik@webrtc.org>
> Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#48544}

Bug: webrtc:479862368
Change-Id: Ie82d63b86651fc55703e38f5017f02c97c286b7b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/506100
Commit-Queue: Kacper Wasniowski <kwasniow@cisco.com>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48769}