feat(mysql): add mysql-rsa feature for non-TLS RSA auth (#4142)

* feat(mysql): add mysql-rsa feature for non-TLS RSA auth

* - Removed unused dependencies from sqlx-mysql (verified no code references).
- Pinned time to =0.3.45 to respect MSRV 1.86 and updated examples to keep the same version explicit.

* revert time pinning in Cargo.toml

* trim Cargo.lock churn and document mysql-rsa for sqlx-cli

---------

Co-authored-by: Austin Bonander <austin.bonander@gmail.com>