Include Sframe library in libwebrtc

# Summary

This CL pulls Chromium's `third_party/sframe` library into WebRTC as a
new third-party dependency. The library provides the core SFrame (RFC
9605) cryptographic primitives — key derivation, AEAD
encryption/decryption, header serialization, and counter management —
that WebRTC's upcoming end-to-end media encryption pipeline will build
on top of.

Bug: webrtc:479862368
Change-Id: Ib72d0cfa6cd5ed24be1e671fde0b124b5284d59d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/474364
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47834}

DEPS

diff --git a/DEPS b/DEPS
index df7fb7d..4c002b3 100644
--- a/DEPS
+++ b/DEPS
@@ -652,6 +652,9 @@
   'src/third_party/re2/src':
     'https://chromium.googlesource.com/external/github.com/google/re2.git@972a15cedd008d846f1a39b2e88ce48d7f166cbd',
 
+  'src/third_party/sframe/src':
+    Var('chromium_git') + '/external/github.com/cisco/sframe' + '@' + 'b14090904433bed0d4ec3f875b9b39f3e0555930',
+
   'src/third_party/r8/cipd': {
       'packages': [
           {