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 it's not terribly useful for profiling *.edit-style methods which do a write: these need a real acting user.

Test Plan:
Ran bin/conduit call --method user.whoami --as epriestley ... with valid, invalid, and no acting users.

$ echo '{}' | ./bin/conduit call --method user.whoami --as epriestley --input -
Reading input from stdin...
{
  "result": {
    "phid": "PHID-USER-icyixzkx3f4ttv67avbn",
    "userName": "epriestley",
    "realName": "Evan Priestley",
...

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19566