Support for google ADK #2240

@jsingh-github, please see this PR as it's very close.
ag-ui-protocol/ag-ui#274

ADK is supported now, so this can be closed 🧹

How do you restore a session from the google ADK backend? It looks like thread persistence is limited to just in InMemoryAgentRunner in copilotkit and that it doesn't attempt to restore a session using a threadId from the GADK service. I've implemented a Google ADK/AG-UI middleware and I can access threads, MESSAGE_SNAPSHOT, STATE_SNAPSHOT, from the ADK endpoints independently, but copilotkit can't or won't.