AgentCoreSessionService extending BaseSessionService for AgentCore Memory

### Is your feature request related to a specific problem?
I'm trying to use AWS AgentCore Memory with an ADK Agent.

### Describe the Solution You'd Like
The session_serivce is a required param in the adk Agent so it would be nice if there was an implementation for AgentCore Memory.

As for the implementation, two things caught my eyes so far:
1. Session
There isn't an exact session translation to agentcore memory so need to think about that.
2. Events and Short-term memory
STM is where events are saved (user message, llm response) in AgentCore memory. In adk they are more fine-grained, but in agentcore they represent a turn - user message is one and what the agent responded is another - maybe we can construct a whole event that consists of all the little ADK events and then flush those to agentcore's short-term memory as, what aws calls, an event.

### Impact on your work
No timeline but would make it easier. This will allow for better integration with AWS AgentCore memory

### Willingness to contribute
Are you interested in implementing this feature yourself or submitting a PR?
Yes, I am happy to work on an implementation myself