Introduce idxv(), like idx()

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10449

Differential Revision: https://secure.phabricator.com/D15347