R1:c7d6fd198cc9

Support "Set X to" as an action in Herald for tokenizer/datasource custom fields

Summary:
See PHI173. Adds custom field support for Herald actions, and implements actions for "Datasource/Tokenizer" fields.

The only action available for now is "set field to...". Other actions ("Add values", "Remove values") might make sense in the future for these fields, but there's currently no use case. For most other field types (text, select, checkbox, etc) only "Set to" makes sense.

Test Plan:
- Added a…