feat: allow custom fields in the Promise status

[richcooper95]

For Resource Requests, we allow custom fields to be written to the status (e.g. via status.yaml in the workflows). We don't currently allow this for Promises: all unknown fields are ignored when the update is made in the update-status script.

This issue is to modify the Promise schema to allow unknown fields to be written to the Status, opening up the possibility of writing custom outputs to the Promise status in the workflows.