MDEV-4307 Support at least 48 utf8 characters in username in server and PAM

Extend plugin auth api to support up to 512 bytes in the user names.
Use the API versioning to support old auth plugins too!

mysql-test/suite/plugins/t/auth_v0100.test

@@ -0,0 +1,26 @@
+--source include/not_embedded.inc
+
+if (!$AUTH_0X0100_SO) {
+  skip No auth_0x0100 plugin;
+}
+
+install soname 'auth_0x0100';
+
+select plugin_name, plugin_type_version from information_schema.plugins where plugin_type='authentication' order by plugin_name;
+
+create user tt identified via auth_0x0100;
+grant select on test.* to zzzzzzzzzzzzzzzz;
+
+--replace_result $MASTER_MYSOCK MASTER_MYSOCK $MASTER_MYPORT MASTER_MYPORT
+--error ER_ACCESS_DENIED_ERROR
+connect (c0,localhost,tt);
+
+grant proxy on zzzzzzzzzzzzzzzz to tt;
+connect (c1,localhost,tt);
+connection c1;
+
+--query_vertical select user(), current_user(), @@external_user
+connection default;
+drop user tt;
+drop user zzzzzzzzzzzzzzzz;
+uninstall plugin auth_0x0100;