Configure context compression per session

**Is your feature request related to a problem? Please describe.**

Currently, context compression is defined on an `App` level. So every session on that app will share the same compression config (including on/off).

I want to control whether compression is turned on/off on a per session bases, based on the user/other metadata.

**Describe the solution you'd like**

Maybe have a lifecycle callback on the `App` layer to configure these options. Something `before_session_start`.

**Describe alternatives you've considered**

Create my own compaction events, using `after_agent` callback.

**Additional context**

I am deploying using Vertex AI AgentEngine, so want to be aligned with the possibilities there.