Implement clientUiCapabilities event from client to agent as part of A2UI

[jacobsimionato]

Note: This mechanism is under discussion with Mandar. See pull request 3 in the A2UI repo (private) for more context. Let's resolve the spec discussion before implementing this.

Dynamic catalogs

• Ability for the client to send dynamic catalogs to the agent, inlined directly into the clientUiCapabilties message

Non-dynamic catalogs

• Ability for a "Catalog" to have URI associated with it which you can set
• Ability for Gen UI Manager to support multiple catalogs. GenUiSurface should probably select the proper Catalog to use for rendering from the GenUiManager host based on the catalog URI in UIDefinition.
• Ability for Gen UI Manager to send a clientUiCapabilities event with it's supported catalogs. Perhaps this should just be an accessor which returns a ClientUiCapabilities instance that apps can plumb as they need to, seeing as it's the apps responsibility to manage connections and sessions with their agents.

[jacobsimionato]

@gspencergoog I added some more context here

[gspencergoog]

Mandar and I decided to add an allowsCustomWidgetLists to the Agent card and default it to "false". We'll change the components list to be a oneOf with either a components list URI or a list of components.