The census_sync field_mapping block in the Terraform provider (sutrolabs/census v0.2.19) can't set a mapping's operation (merge/overwrite). The API supports it for array fields such as HubSpot association fields (association_ids:HUBSPOT_DEFINED:279). We need merge so a sync never removes associations created outside it.

In v0.2.19:
- Mappings Terraform creates are stored with operation null.
- Every apply re-sends all mappings without an operation, which resets a merge set in the UI. This happens even with ignore_changes = [field_mapping].
- The provider doesn't read the operation back, so the reset never shows in a plan.

Requests:
1. Add an optional operation argument to field_mapping, sent on create and update and read back. Is there a timeline?
2. Document what a null operation does for HubSpot associations. Does it remove associations that aren't in the source, including ones created outside the sync?