Add structured reporting diff to APIGatewayAPI (#6781)

### BRIEF Change description

Fixes #6530

#### WHY do we need this change?

Add structured reporting diff to the controller in
`pkg/controller/direct/apigateway/api_controller.go`.
The `structuredreporting.ReportDiff` should be used in the `Update`
method of the adapter to report which fields are being updated.
This helps in debugging reconciliation loops and provides better
visibility into what changed.

#### Special notes for your reviewer:

#### Does this PR add something which needs to be 'release noted'?
```release-note
NONE
```

#### Additional documentation e.g., references, usage docs, etc.:
```docs
NONE
```

#### Intended Milestone
- [ ] Reviewer tagged PR with the actual milestone.

### Tests you have done

- [ ] Run `make ready-pr` to ensure this PR is ready for review.
- [ ] Perform necessary E2E testing for changed resources.

Author: cheftako

pkg/controller/direct/apigateway/api_controller.go

@@ -39,6 +39,7 @@ import (
 	"github.com/GoogleCloudPlatform/k8s-config-connector/pkg/controller/direct/directbase"
 	"github.com/GoogleCloudPlatform/k8s-config-connector/pkg/controller/direct/registry"
 	"github.com/GoogleCloudPlatform/k8s-config-connector/pkg/label"
+	"github.com/GoogleCloudPlatform/k8s-config-connector/pkg/structuredreporting"
 )
 
 func init() {
@@ -185,6 +186,12 @@ func (a *apiAdapter) Update(ctx context.Context, updateOp *directbase.UpdateOper
 		log.V(2).Info("no field needs update", "name", a.id)
 		updated = a.actual
 	} else {
+		report := &structuredreporting.Diff{Object: updateOp.GetUnstructured()}
+		for _, path := range paths {
+			report.AddField(path, nil, nil)
+		}
+		structuredreporting.ReportDiff(ctx, report)
+
 		resource.Name = a.id.String() // we need to set the name so that GCP API can identify the resource
 		req := &pb.UpdateApiRequest{
 			Api:        resource,