Migrate AgentCore memory integration to new Session API

[ashakirin]

Spring AI introduced new Session API in favorite to old one (ChatMemory, ChatMemoryRepository).
The old APIs will deprecated and disappear in 3.0.0.
We have to migrate our AgentCore memory integration to new Session API

[dgomesbr]

@ashakirin I'm gating the change behind agentcore.memory.session.enabled until we decide to deprecate?
I can remove that if you want breaking changes.

You can assigned WIP and we can discuss in here.