Full support for A2A on agent_runtime

[sushicw]

Some aspects of A2A agents on Agent Runtime are not yet fully supported.

A couple of (non-exhaustive) specifics:

1. agents-cli scaffolded agents are scaffolded using adk-a2a will be using inmemory session and task store, rather than a service.
2. The Agent Runtime Console Playground does not currently support A2A agents.

Internal: b/503728939