Allow specifying the leeway as a service parameter.

The Firebase JWT SDK supports a leeway parameter to correct time drifts between servers.

/**
 * You can add a leeway to account for when there is a clock skew times between
 * the signing and verifying servers. It is recommended that this leeway should
 * not be bigger than a few minutes.
 *
 * Source: http://self-issued.info/docs/draft-ietf-oauth-json-web-token.html#nbfDef
 */
JWT::$leeway = 60; // $leeway in seconds
$decoded = JWT::decode($jwt, $key, array('HS256'));

Perhaps we can have a container parameter that sets this and is invoked by the transcoder?