feat: provide access to session events from executor callbacks (#521)

* provide access to session events from executor callbacks

* gemini nitpicks

* test cleanup

Update genai to v1.40.0 (#424)

feat: pass event actions through a2a (#304)

pass Escalate and TransferToAgent through a2a

Initial release of the Agent Development Kit (ADK)

Agents: LLMAgent, Workflow Agents, Custom Agents.
A2A support: expose ADK agents via A2A server or connect to A2A via RemoteAgent.
Tool support: function tool, MCP tools, long-running tools and other.
Callbacks for agent, LLM and tools.
LLMs: GeminiAPI and VertexAI
Session service: in-memory and persistent implementations.
Artifact service: in-memory and Google Cloud Storage.
Memory service: in-memory implementation.
REST API server + WebUI launchers.