Support gradual rollout of new models with agent registry

Right now, the template in the prompt registry for AIGW determines the model version used (see example )

Before moving code generation to the AIGW, we used feature flags to rollout new models gradually to users so that we could easily revert if there was an issue.

We need the ability to do this with the prompt registry version of code generation, and I am sure other tools could benefit from this functionality.