Deployment: Lumen microservice gRPC API

[q3k]

We need to expose the current Lumen web panel consumer as a gRPC API for use by the BMaaS control plane.

This involves:

1. Designing the API, probably making it generic enough to not be Lumen-specific, and pushing that to the monorepo.
2. Actually implementing this spec inside the Lumen codebase (outside monorepo)

This can then be consumed by the mini-Shepherd.