.NET: [Feature]: Provide a way to specify the lifetime of AgentSessionStore

### Description

Now that `AddAIAgent` allows specifying the lifetime of agents (https://github.com/microsoft/agent-framework/issues/4457), it would be useful to have the same capability for the `WithSessionStore` extension method. This could be useful, for example, when using a `DbContext` for session management.

### Language/SDK

.NET