Fix a bug with setting custom PHID list field values via Conduit and prepare for bulk edits

Summary:
Ref T13025. Custom field transactions work somewhat unusually: the values sometimes need to be encoded. We currently do not apply this encoding correctly via Conduit.

For example, setting some custom PHID field to ["PHID-X-Y"] fails with a bunch of JSON errors.

Add an extra hook callback so that EditTypes can apply processing to transaction values, then apply the correct CustomField processing.

This only affects Conduit. In a future diff, this also allows bulk edit of custom fields to work correctly.

Test Plan: Added a custom field to Maniphest with a list of projects. Used Conduit to bulk edit it (which now works, but did not before). Used the web UI to bulk edit it.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13025

Differential Revision: https://secure.phabricator.com/D18876