Add "modifyExtensionField" utility function

As calling $updater->modifyField will do the action immediately (not queued),
this may be undesired.

So this is a shortcut/utility function like addExtensionField/addExtensionTable etc,
instead of the developer calling addExtensionUpdate( array() ) themselves.

Change-Id: Ie94f2b42679062c6e106ae560faab26818a9b15c
Merge "Add "modifyExtensionField" utility function"