Introduce idxv(), like idx()
Ref T10449. The new Conduit APIs are more powerful/flexible, but also more annoying to read data from since things are buried several levels deep.
Introduce idxv() to make it easier to do $conduit_result['data']['attachments']['fields']['name'] and similar safely without checking every single value.
Test Plan: See other diffs associated with T10449.
Reviewed By: chad
Maniphest Tasks: T10449
Differential Revision: https://secure.phabricator.com/D15347