X.509 signatures and verification

[kaylendog]

This PR implements X.509 signatures and verification, and is a refactored/linear version of the changes in andyrich/x509.

This is a big PR, so I have compressed the original changes into several discrete commits, each of which should be independently reviewable:

• The X509Signature struct, and the two core traits, X509Signer and X509Verifier.
• A Rust-native implementations of the above two traits, threaded through the existing crypto store.
• Signing and verifying the master signing key (MSK) using X.509.
• FFI boilerplate, since some X.509 functionality needs to be implemented natively.
• Integration tests

rsa vulnerability

The crate we use for constructing signature definitions, rsa, has a vulnerability in its padding algorithm (see RustCrypto/RSA#626). However, this only affects RSA private key operations, which we don't perform at all. Thus, we should be able to safely ignore the attached CI failures.

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:

[codecov]

Codecov Report

❌ Patch coverage is 91.17371% with 94 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.95%. Comparing base (7bf142a) to head (99bc3da).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...s/matrix-sdk-crypto/src/x509/raw_x509_signature.rs	82.44%	15 Missing and 28 partials ⚠️
crates/matrix-sdk-crypto/src/x509/x509_verify.rs	94.47%	11 Missing and 1 partial ⚠️
...trix-sdk-crypto/src/x509/rust_raw_x509_verifier.rs	83.07%	8 Missing and 3 partials ⚠️
crates/matrix-sdk-crypto/src/x509/x509_signer.rs	83.67%	6 Missing and 2 partials ⚠️
...matrix-sdk-crypto/src/x509/rust_raw_x509_signer.rs	81.08%	3 Missing and 4 partials ⚠️
crates/matrix-sdk-crypto/src/olm/account.rs	72.72%	1 Missing and 2 partials ⚠️
crates/matrix-sdk-crypto/src/machine/mod.rs	95.00%	0 Missing and 2 partials ⚠️
crates/matrix-sdk-crypto/src/olm/signing/mod.rs	95.65%	0 Missing and 2 partials ⚠️
...-sdk-crypto/src/types/signatures/x509_signature.rs	92.59%	0 Missing and 2 partials ⚠️
crates/matrix-sdk-base/src/client.rs	95.00%	0 Missing and 1 partial ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6727      +/-   ##
==========================================
+ Coverage   89.92%   89.95%   +0.03%     
==========================================
  Files         398      405       +7     
  Lines      111157   112187    +1030     
  Branches   111157   112187    +1030     
==========================================
+ Hits        99954   100921     +967     
- Misses       7400     7427      +27     
- Partials     3803     3839      +36     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing kaylendog/x509/impl (99bc3da) with main (7bf142a)}