Reland "Sframe encryptor implementation"

This reverts commit 3590bc3fc94418ecddf2a2cf392e19f8a1a17a05.

Reason for revert: Dependency added downstream

Original change's description:
> Revert "Sframe encryptor implementation"
>
> This reverts commit 4820cf11c56af592ea4ed02cbce7c0b1a7656fbe.
>
> Reason for revert: Breaks downstream projects due to new third party dependency.
>
> Original change's description:
> > Sframe encryptor implementation
> >
> > This CL introduces the concrete Sframe encryptor that sits between the
> > public key-management API and the media pipeline, implementing the
> > sender-side wiring for Sframe end-to-end encryption.
> >
> > # High-level design
> > The implementation follows a three-layer split that matches the existing
> > WebRTC layering and respects the api/ visibility rules: 1. Public API
> > layer (api/sframe/) - A narrow, stable interface that exposes only key
> > management to applications: setting an encryption key on the sender
> > side.
> >
> > 2. Internal media interface layer (modules/sframe/) - Extends the public
> > key-management interface with the Encrypt and "max ciphertext size"
> > entry points used by the media pipeline. This is the surface the media
> > channel speaks to; application code never sees it.
> >
> > 3. Concrete impl layer (modules/sframe/) - Implements the media
> > interface on top of the cisco/sframe library.
> >
> > Splitting key management from frame protection lets the public handle
> > expose only the small surface applications need, while the media
> > pipeline talks to a richer interface internally.
> >
> > Bug: webrtc:479862368
> > Change-Id: Id93dba5874b6f5b1f32723755dd404af202f6ecf
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/481301
> > Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> > Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
> > Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
> > Cr-Commit-Position: refs/heads/main@{#48065}
>
> Bug: webrtc:479862368
> Change-Id: Icef4fe1ce6e391b045f7b94087ee419b7899a96c
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/485200
> Commit-Queue: Johannes Kron <kron@webrtc.org>
> Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Björn Terelius <terelius@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#48077}

Bug: webrtc:479862368
Change-Id: Ie9dae7c1e52f459b5b5b762937febadab9a33588
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/488000
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#48170}