Support caching_sha2_password

MySQL 8.0 adds `caching_sha2_password` that "implements SHA-256 authentication (like sha256_password), but uses caching for better performance and has additional features for wider applicability." https://dev.mysql.com/doc/refman/8.0/en/caching-sha2-pluggable-authentication.html

(Support for `sha256_password` was added in #281.)