Avoid "Unknown Object (????)" for custom Select field values in Herald editor

Summary:
Custom Select field values are rendered as Unknown Object (????) when set as Herald rule conditions and editing the Herald rule.
This is unhelpful; at least display their key value.

Closes T15860

Test Plan: Create a "type": "select" custom field via /config/edit/maniphest.custom-field-definitions/, set up a Herald rule with a condition based on that custom field, edit the Herald rule.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15860

Differential Revision: https://we.phorge.it/D25696