Allow to specify an auth plugin

[julienschmidt]

Allow users to specify a auth plugin in the DSN which is then used initially instead of the server's default auth plugin. Doing so can eliminate 1 roundtrip during the handshake.

Example: The servers default auth plugin is caching_sha2_password but the requested user is authenticated via PAM. Thus using mysql_cleartext_password or dialog (#803) must be used on the client-side. Currently the driver attempts authentication using caching_sha2_password first. The server then tells the driver to switch to mysql_cleartext_password or dialog.

[methane]

I'm not sure, but client may be not able to override first auth method specified by server.

https://dev.mysql.com/doc/internals/en/auth-phase-fast-path.html

Assume that client wants to log in as user U and that user account uses authentication method M. The fast authentication path is used if both client and server used method M to generate authentication data in the initial handshake.