Assign a facility credentialing workflow to a user

Creates an assignment record for the specified facility credentialing workflow, associating it with the given user and appending an entry to the assignment history. When to use: Use to assign (or reassign) a single workflow to one user. To assign multiple workflows at once, use POST /facility-credentialing-workflows/assignments. Preconditions: The caller must hold the FACILITY_CREDENTIALING_WORKFLOW_ASSIGN or ASSIGN_SELF permission. The target user must have the required permissions to be assigned; if not, 422 Unprocessable Entity is returned — select a different assignee or ensure the target user's permissions are elevated. Side effects: An assignment history entry (FacilityAssignmentHistoryResponse) is created. Returns: 201 Created with the new FacilityAssignmentHistoryResponse in the body.