R1:a26cf20dd128

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…