[AC-1713] [Flexible collections] Add feature flags to server (#3334)

* Add feature flags for FlexibleCollections and BulkCollectionAccess

* Flag new routes and behaviour

---------

Co-authored-by: Rui Tomé <108268980+r-tome@users.noreply.github.com>

src/Api/Controllers/OrganizationsController.cs

@@ -765,6 +765,7 @@ public class OrganizationsController : Controller
     }
 
     [HttpPut("{id}/collection-management")]
+    [RequireFeature(FeatureFlagKeys.FlexibleCollections)]
     public async Task<OrganizationResponseModel> PutCollectionManagement(Guid id, [FromBody] OrganizationCollectionManagementUpdateRequestModel model)
     {
         var organization = await _organizationRepository.GetByIdAsync(id);