feat(backend): add worker drain Admin API - #11118

PR issue link: proposal only

Found #11173, which is a Dynamo Enhancement Proposal.
A DEP is a proposal umbrella: it stays open across many pull requests, so
it records what the work is part of and not what this change is. Keep the
reference and link the work as well.

Every PR needs a linked issue so the work is traceable to a tracked task.
Link one of the following and re-run the check (editing the PR description re-triggers it):

• A Linear issue, for example Closes DYN-1234 in the description, or the
  issue ID in the branch name (user/dyn-1234-description).
• A GitHub issue: Fixes #123 to close it, or a non-closing form like
  Part of #123 for an issue that outlives the PR. Issues in sibling
  repositories in the org count too, for example
  ai-dynamo/enhancements#12.

If no issue exists yet, create one first and start the work from it.
This check is required: release/1.7.0 has been cut.