[ECS] [request]: Add validation to check if default security group exists when CreateService API is run without any security groups

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**Tell us about your request**
There should be a validation in CreateService API when no security group is specified in the request and a default security group doesn't exist.

**Which service(s) is this request for?**
Fargate - ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
As per [ECS documentation](https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_AwsVpcConfiguration.html#API_AwsVpcConfiguration_Contents) , when there is no security group defined in the `CreateService` API call in ECS, it uses the default security group.

In some cases, if there is no default security group present, the service is still created, but without any security group, thus tasks fail to start.

If there is a validation that verifies if the CreateService API doesn't have a security group in the AwsVpcConfiguration and there is no default security group in the VPC, then the service does not get created, it would be very useful and make the service more robust to user errors.

**Are you currently working around this issue?**
How are you currently solving this problem?
Manually update the service to add a security group.

**Additional context**
Anything else we should know?

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

The issue concerns the ECS Fargate CreateService API and its AwsVpcConfiguration, with behavior described in the linked ECS API documentation. No repository files, tests, or implementation entry points are mentioned; first clarify where this service-side validation could be contributed, and consider the request done when a service without specified security groups is rejected if the VPC has no default security group.

Written by the indexing model from the issue text.