R1:3ca3f09a1aa6

Add an "--as" flag to "bin/conduit call ..." to improve flexibility and ease of profiling

Summary:
Ref T13164. In PHI801, an install reported a particular slow Conduit method call.

Conduit calls aren't easily profilable with normal tools (for example, `arc call-conduit --xprofile ...` gives you a profile of the //client//). They can be profiled most easily with `bin/conduit call ... --xprofile`.

However, `bin/conduit call` currently doesn't let you pick a user to execute the command on behalf of, so…