feat: allow machine user id to be set during creation (#8265)

# Which Problems Are Solved
Solves the problem described in #8264.

# How the Problems Are Solved

Added a UserID field which can be set during Machine User creation.

# Additional Changes

Added addition unit and integration tests to cover the cases where a
UserID field is present.

# Additional Context

- Closes #8264

Co-authored-by: Stefan Benz <46600784+stebenz@users.noreply.github.com>

internal/api/grpc/management/user_converter.go

@@ -133,8 +133,14 @@ func ImportHumanUserRequestToDomain(req *mgmt_pb.ImportHumanUserRequest) (human
 }
 
 func AddMachineUserRequestToCommand(req *mgmt_pb.AddMachineUserRequest, resourceowner string) *command.Machine {
+	userId := ""
+	if req.UserId != nil {
+		userId = *req.UserId
+	}
+
 	return &command.Machine{
 		ObjectRoot: models.ObjectRoot{
+			AggregateID:   userId,
 			ResourceOwner: resourceowner,
 		},
 		Username:        req.UserName,