[A2UI] Adopt "action" format which can refer to the data model for context

[jacobsimionato]

See internal doc which describes the schema of an action: https://docs.google.com/document/d/1SppqNwHytVIUp7o8x5py5Z7AIiMdz-pzgC0SWh9qfkE/edit?tab=t.cfbq3pyhe3w#bookmark=id.ir97t6sc6920

See the spec of the A2UI format at go/a2ui-repo (private repo) e.g. see the section "Resolving the action.context"

Some parts of this:

• Add a utility to a2ui_schema.dart based on the schema above
• Update any relevant widgets (probably just the ElevatedButton and InputGroup widget for now) to support this, so that they can refer to data model locations which add context to the action. Other widgets that support actions e.g. TravelCarousel etc don't need to have configurable context I think - they can just trigger actions.
• Update central action handling logic in GenUiManager so that rather than including the entire data model state (which I'm doing in [GULF] Adopt the GULF concept of data model #345 as a stop gap), it just includes the values that are referred to as context.

[jacobsimionato]

Completed in #370