Workhorse: Act as a proxy to Duo Workflow

What does this MR do and why?

Implementation of this idea: Duo Workflow ADR: Workhorse as a DW Service proxy (gitlab-com/content-sites/handbook!13685 - merged).

As a result, implementing Web Agentic Duo Chat is possible: Draft: Web agentic chat poc (!193151 - closed)

Related issue: Workhorse WS handler that proxies gRPC connecti... (#547502 - closed)

Current implementation:

This MR allows the following: