Decide the next steps for SubstrateAgent

[rakyll]

We have implemented an experimental SubstrateAgent yet we don't know if this is a reasonable thing to support from the controller. SubstrateAgent allows creating and deleting actor, and runs a sub agent as an isolated actor.

We can make Substrate support the implementation detail of agent builders, and remove SubstrateAgent from the AX repo.

[rakyll]

cc @dberkov

[dberkov]

According to the https://cloud.google.com/blog/products/containers-kubernetes/bringing-you-agent-sandbox-on-gke-and-agent-substrate blog post, the substrate layer occupies a specific high-level position within the application architecture. This layer is designed to remain abstracted from the end user; instead, an agent application layer is constructed on top of it.

Consequently, for the Ax system, users should interact with the Ax platform itself rather than the underlying substrate directly. This architecture implies that the Ax system requires a front-end to intercept and process customer requests before forwarding them to the substrate layer.

[rakyll]

This sounds good. Let's improve the SubstrateAgent and move it out of the experimental stage. We'd enable SubstrateAgent to provide auto resumption by default too. This path will be useful for writing stateless long running agents.