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