Add authentication plugin name to authenticator API

The authenticators can now declare the authentication plugin name. Right
now this is only relevant for MySQL authentication but for example the
HTTP module could implement both Basic and Digest authentication.

server/modules/include/mysql_client_server_protocol.h

@@ -79,6 +79,8 @@
 #define GW_MYSQL_SCRAMBLE_SIZE 20
 #define GW_SCRAMBLE_LENGTH_323 8
 
+#define DEFAULT_AUTH_PLUGIN_NAME "mysql_native_password"
+
 /** Maximum length of a MySQL packet */
 #define MYSQL_PACKET_LENGTH_MAX 0x00ffffff