Support AuthSwitchRequest to switch authentication plugin

[clue]

This changeset adds support for the AuthSwitchRequest to switch authentication plugins. The MySQL server can send this packet to ask the client to use another authentication method (https://dev.mysql.com/doc/dev/mysql-server/latest/page_protocol_connection_phase_packets_protocol_auth_switch_request.html).

For instance, this is required to make Amazon RDS work in my tests. In my tests, the server first requests mysql_native_password which is then fulfilled by the client, and next switches to caching_sha2_password afterwards, asking the client to authenticate again. While I agree this may look strange, it may make sense depending on server configuration internals and is completely in line with the MySQL protocol specification.

With these changes applied, I can successfully connect to my Amazon RDS server and process any queries as usual. Prior to applying these changes, I would get a protocol exception instead:

Connection to mysql://user:***@example.amazonaws.com:3306/example failed during authentication: Unexpected protocol error, received malformed packet with 39 unknown byte(s)

The affected code has 100% code coverage and has been tested against a number of MySQL server versions. The tests confirm it should continue to work for legacy servers not using authentication plugins or newer MySQL server versions using the existing mysql_native_password authentication or newer caching_sha2_password authentication. As discussed in #196 and elsewhere, I'll follow-up with a separate PR to add more MySQL server versions to our test matrix, but I've yet to find a combination that requires the AuthSwitchRequest.

Builds on top of #207 and #206

[WyriHaximus]

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