This extracts a plugin name from the server and exposes this as
connection.get_plugin_name().
The client side here adds capabilities PLUGIN_AUTH and
PLUGIN_AUTH_LENENC_CLIENT_DATA.
Because of this the HandshakeResponse response has been altered to use
the server_capabilities a bit mroe strictly.
The immediate upshot is that plugins like unix_socket are immediately
supported where they previously would of returned and error as without
PLUGIN_AUTH being advertised on the client side, the server only accepts
mysql_native_password and mysql_old_password.
To support other plugins some more work is needed to implement these.
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