Support for MSC2762 update_state widget action when running as embedded client

[hughns]

A new widget action called update_state was recently added to MSC2762.

The embedded RoomWidgetClient should use this capability where available in order to get saner room state semantics.

n.b. The was a previous PR to implement this in #4574 but then reverted in #4656 as it was not backwards compatible.

The implementation should maintain compatibility so that embedded RoomWidgetClient will continue to work with widget container/hosts irrespective if they support the update_state action.