X.509 Preparation

[kaylendog]

This PR extracts three of the more general changes made in andyrich/x509 (the X.509 development branch) and applies them independently. In summary:

• Adjusts Sugnatures::add_signature to accept any type implementing Into<Signature> - done in advance of adding X.509-specific signature structs;
• Makes bootstrap_cross_signing falliable, since X.509 signature setup can fail;
• Inlines Account::sign_master_key, as this is only used in one place, and will be touched by X.509 anyway.

PR checklist:

• I've documented the public API changes in the appropriate changelog files (see Writing changelog entries).
• This PR was made with the help of AI.

Signed-off-by: Skye Elliot actuallyori@gmail.com

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.91%. Comparing base (c9c4b26) to head (63b4987).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/error.rs	0.00%	5 Missing ⚠️
crates/matrix-sdk-crypto/src/identities/user.rs	85.71%	0 Missing and 2 partials ⚠️
crates/matrix-sdk-crypto/src/olm/account.rs	71.42%	0 Missing and 2 partials ⚠️
crates/matrix-sdk-crypto/src/machine/mod.rs	50.00%	0 Missing and 1 partial ⚠️
crates/matrix-sdk-crypto/src/olm/signing/mod.rs	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6715      +/-   ##
==========================================
- Coverage   89.92%   89.91%   -0.02%     
==========================================
  Files         398      398              
  Lines      111154   111157       +3     
  Branches   111154   111157       +3     
==========================================
- Hits        99959    99947      -12     
- Misses       7401     7412      +11     
- Partials     3794     3798       +4     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing kaylendog/x509/preparation (63b4987) with main (c9c4b26)}

[andybalaam]

Looks good. A couple of minor comments.