[Workhorse] Handle runHTTPRequest action in duo workflow handler

Overview

Workhorse WS handler that proxies gRPC connecti... (#547502 - closed) and Workhorse: Act as a proxy to Duo Workflow (!193149 - merged) introduces Workhorse as a proxy to Duo Workflow Service

When the service needs to perform API request to Gitlab, it performs a request directly to the instance

Problem

It's challenging from the Security point of view to perform direct requests to arbitrary self-managed instance, so we still need to proxy the requests via user machine.

Even if we implement direct requests to the instances, it still may be a problem to the customers with closed networks that doesn't allow any incoming requests.

Proposal

Handle runHTTPRequest action in Workhorse and perform Rails requests directly from there

Open questions

If we need to wrap such requests into a composite identity, what would be the solution