Report deployment status in Admin APIs and enable safely deleting drained deployments

[pcholakov]

Currently, the list+get deployment Admin APIs do not report active status for a deployment the way the CLI does. Further, deployments can only be force-deleted via the Admin delete API. This leads to a confusing operator experience with the potential pitfall of being able to accidentally remove an active deployment. Drained deployments are safe to delete, but they can neither be easily discovered (outside of the CLI) nor safely deleted (as the DELETE operation requires force=true).

1. Add a deployment status attribute with possible values Active | Drained to at least the Get Deployment Admin API, and ideally the List Deployments API too.
2. Allow deleting fully-drained deployments without requiring force=true.

Implementing both of these would also enable an easy way to automatically prune fully drained deployments in bulk. This would be particularly helpful with Lambda deployments where you can easily accumulate large numbers of inactive deployments on older versions of a function.

Related: #1377