Update an existing Shared Custom Field Association
You can change the `required` or the `editable_with_roles` attributes. Everything else cannot be changed.
The `object_type` in the URL can be either:
- `lead` when editing the association with the Lead object.
- `contact` when editing the association with the Contact object.
- `opportunity` when editing the association with the Opportunity object.
- `custom_activity_type/<catype_id>` when editing the association with a specific
Custom Activity Type.
- `custom_object_type/<cotype_id>` when editing the association with a specific
Custom Object Type.