Make API action=tokens extendable

Make API action=tokens extendable

LocalSettings snippet for testing:

$wgHooks['ApiTokensGetTokenTypes'][] = function( &$hookTypes ) {
	$hookTypes['foo'] = function() { return 'bar'; };
	return true;
};

Change-Id: Idf1f4379e677d21059c1b4e12f80e9d2bafb9897