feat(mysql): allow prebuilt rustls TLS config

[mingley]

Summary

• add a tls_server_name override to MySqlConnectOptions
• use the override for TLS SNI and hostname verification during MySQL TLS upgrade
• add a small unit test + docs for the new option

Rationale

This is a small, additive, opt-in change to fix VerifyIdentity when the TCP host differs from
what the server certificate expects (e.g., HAProxy endpoints). It does not alter existing behavior
unless the new option is explicitly set.

Testing

• cargo fmt
• cargo clippy -p sqlx-mysql -- -D warnings
• cargo test -p sqlx-mysql

[mingley]

Superseded by #4130