Add Standard Custom Fields to Item List

Summary:
Allow "Standard" (config-based") custom fields to be displayed in search-results.

Depends on D25548. Ref T15750.

Test Plan: Set maniphest.custom-field-definitions to the value of P32, and start playing with custom values on tasks.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: 20after4, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15750

Differential Revision: https://we.phorge.it/D25549