feat(mysql): make rsa dependency optional (RUSTSEC-2023-0071)

[cmaher]

Summary

• Makes the rsa crate an opt-in feature of sqlx-mysql to address RUSTSEC-2023-0071 (Marvin Attack timing side-channel, no patched release)
• Gates RSA-based auth (sha256_password/caching_sha2_password over non-TLS) behind features = ["rsa"]; returns a clear error when the feature is absent
• CI and test docker-compose updated to use mysql_native_password to avoid requiring RSA

Copied entirely from launchbadge#4186

Test plan

• Verify cargo build succeeds without rsa feature (TLS-only usage)
• Verify cargo build --features rsa succeeds and non-TLS RSA auth works
• Verify cargo audit no longer flags RUSTSEC-2023-0071 when rsa feature is disabled

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