feat(mysql): add aws-lc-rs backend for RSA auth and FIPS TLS support

[dertin]

Does your PR solve an issue?

fixes #4082

Is this a breaking change?

Potentially. Public APIs are unchanged and defaults remain the same, but --all-features no longer works for sqlx-mysql because mutually exclusive RSA backend features were added. Build/CI scripts that relied on --all-features must select a backend explicitly.

Summary

Add aws-lc-rs RSA auth backend for MySQL/MariaDB with a FIPS option, plus a rustls FIPS TLS feature.
Refactor MySQL RSA auth backends to prefer aws-lc-rs when selected, and expose new facade features.
Update docs, changelog, and CI/test tooling to cover the new feature matrix and FIPS prerequisites.

Testing

- cargo check -p sqlx-mysql --no-default-features --features rsa-aws-lc-rs
- cargo check -p sqlx-mysql --no-default-features --features rsa-aws-lc-rs-fips
- cargo test -p sqlx-mysql --no-default-features --features rsa-aws-lc-rs --lib
- cargo test -p sqlx-mysql --no-default-features --features rsa-aws-lc-rs --doc --features sqlx/mysql-auth-rsa-aws-lc-rs
- cargo check -p sqlx --no-default-features --features mysql-auth-rsa-aws-lc-rs,runtime-tokio,tls-native-tls
- cargo check -p sqlx --no-default-features --features mysql-auth-rsa-aws-lc-rs-fips,runtime-tokio,tls-rustls-aws-lc-rs-fips
- cargo test -p sqlx-core --no-default-features --features _tls-rustls-aws-lc-rs
- cargo test -p sqlx-core --no-default-features --features _tls-rustls-aws-lc-rs-fips
- cargo test -p sqlx-core --no-default-features --features _tls-rustls-ring-webpki
- python tests/x.py --fips

[dertin]

Hi @abonander

This PR adds two MySQL RSA auth backend features:

• mysql-auth-rsa-aws-lc-rs (non‑FIPS, replaces rsa for caching_sha2_password/sha256_password when TLS is off)
• mysql-auth-rsa-aws-lc-rs-fips (FIPS module, requires Go/CMake on supported targets)

On the TLS side, this PR adds tls-rustls-aws-lc-rs-fips to expose the FIPS rustls provider.
The FIPS and non‑FIPS (tls-rustls-aws-lc-rs) variants are mutually exclusive.

If you'd prefer a different feature layout or naming, I'm happy to adjust.