Lift Diffusion Conduit call proxying to the root level of Conduit

Summary:
Ref T13552. Some Diffusion conduit calls may only be served by a node which hosts a working copy on disk, so they're proxied if received by a different node.

This capability is currently bound tightly to "DiffusionRequest", which is a bundle of context parameters used by some Diffusion calls. However, call proxying is not fundamentally a Diffusion behavior.

I want to perform proxying on a "*.search" call which does not use the "DiffusionRequest" parameter bundle. Lift proxying to the root level of Conduit.

Test Plan: Browsed diffusion in a clusterized repsository.

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21442