Feature: Add a storage abstraction for persisting task objects and artifacts to other storage locations like redis

[Stevenic]

Is your feature request related to a problem? Please describe.
You can't scale out an Agent because tasks and artifacts are only ever stored in memory.

Describe the solution you'd like
There should be a storage abstraction added that lets you replace the in-memory based storage of tasks and artifacts with an external storage container like redis. I would also add the ability for the Agent developer to store their own state alongside the tasks and artifacts.

[Stevenic]

The CRUD operations around artifacts should also be abstracted:

https://github.com/AI-Engineer-Foundation/agent-protocol-sdk-js/blob/main/src/agent.ts#L355

I may want to store them in some form of blob storage versus the local disk.