Base model management in Composer for Orchestrator

[boydc2014]

To support orchestrator as a LU alternative, once we bring in orchestrator lib in composer, we need to do a few things to manage the base models,

1. pick a location to store the base models
2. query the base model list and get an available model list, user should be able to select from the list.
3. download and save the model in the picked location. The timing for downloading the base model is the first time a specific base model version.

This depends on #4132