feat(crypto): Introduce ForwarderData for session forwader info.

[kaylendog]

• Introduces a new enum ForwarderData as a wrapper for valid variants of SenderData for which we can accept key bundles under MSC4268.

• Converts forwarder_data in InboundGroupSession and PickledInboundGroupSession to use Option<ForwarderData> over Option<SenderData>.

• Public API changes documented in changelogs (optional)

[codspeed-hq]

CodSpeed Performance Report

Merging #5980 will not alter performance

_{Comparing kaylendog/history-sharing/forwarder-info (cce6533) with main (c3c367c)}

Summary

✅ 50 untouched

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.54%. Comparing base (759c5a9) to head (cce6533).
⚠️ Report is 11 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...dk-crypto/src/olm/group_sessions/forwarder_data.rs	78.57%	8 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5980      +/-   ##
==========================================
- Coverage   88.55%   88.54%   -0.01%     
==========================================
  Files         363      364       +1     
  Lines      104020   104075      +55     
  Branches   104020   104075      +55     
==========================================
+ Hits        92114    92155      +41     
- Misses       7528     7538      +10     
- Partials     4378     4382       +4     

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[richvdh]

LGTM otherwise