Allow extensions to add params to the update.php maintenance script

Allow extensions to add params to the update.php maintenance script

T110209 caused maintenance scripts to fail on unknown parameters,
which is normally desirable.  However, some extensions (notably
SemanticMediaWiki) need to support additional params and had no
way to add them to the list of expected parameters.  It will
now be possible them to update.php via a new hook:
MaintenanceUpdateAddParams.

Bug: T213893
Change-Id: Ia40949ccb2f32090f21e0f3f7e5b9c4aef322330