feat(actionsv2): Propagate request headers in actions v2 (#10632)

# Which Problems Are Solved

This PR adds functionality to propagate request headers in actions v2.

# How the Problems Are Solved
The new functionality is added to the`ExecutionHandler` interceptors,
where the incoming request headers (from a list of allowed headers to be
forwarded) are set in the payload of the request before calling the
target.

# Additional Changes
This PR also contains minor fixes to the Actions V2 example docs.

# Additional Context
- Closes #9941

---------

Co-authored-by: Marco A. <marco@zitadel.com>
(cherry picked from commit 51e12e224d)

internal/api/grpc/action/v2/integration_test/execution_target_test.go

@@ -76,7 +76,15 @@ func TestServer_ExecutionTarget(t *testing.T) {
 				targetCreated := instance.CreateTarget(ctx, t, targetCreatedName, targetCreatedURL, target_domain.TargetTypeCall, false)
 
 				// request received by target
-				wantRequest := &middleware.ContextInfoRequest{FullMethod: fullMethod, InstanceID: instance.ID(), OrgID: orgID, ProjectID: projectID, UserID: userID, Request: middleware.Message{Message: request}}
+				wantRequest := &middleware.ContextInfoRequest{
+					FullMethod: fullMethod,
+					InstanceID: instance.ID(),
+					OrgID:      orgID,
+					ProjectID:  projectID,
+					UserID:     userID,
+					Request:    middleware.Message{Message: request},
+					Headers:    map[string][]string{"Content-Type": {"application/grpc"}, "Host": {instance.Host()}},
+				}
 				changedRequest := &action.GetTargetRequest{Id: targetCreated.GetId()}
 				// replace original request with different targetID
 				urlRequest, closeRequest, calledRequest, _ := integration.TestServerCallProto(wantRequest, 0, http.StatusOK, changedRequest)
@@ -144,6 +152,7 @@ func TestServer_ExecutionTarget(t *testing.T) {
 					UserID:     userID,
 					Request:    middleware.Message{Message: changedRequest},
 					Response:   middleware.Message{Message: expectedResponse},
+					Headers:    map[string][]string{"Content-Type": {"application/grpc"}, "Host": {instance.Host()}},
 				}
 				// after request with different targetID, return changed response
 				targetResponseURL, closeResponse, calledResponse, _ := integration.TestServerCallProto(wantResponse, 0, http.StatusOK, changedResponse)