Support OpenID Connect Pluggable Authentication

A new feature in MySQL Server Enterprise Edition 9.1 is [OpenID Connect Pluggable Authentication](https://dev.mysql.com/doc/refman/9.1/en/openid-pluggable-authentication.html).

On the client side, this seems to be supported via a new `OpenidIdentityToken` connection option: https://dev.mysql.com/doc/relnotes/connector-net/en/news-9-1-0.html