feat(ecs): allow adding security group(s) after construction via connections.addSecurityGroup()

[hamilton-earthscope]

Allow easily adding Security Groups to ECS Services after the L2 Service has already been instantiated by using the service's .connections.addSecurityGroup().

Issue # (if applicable)

Closes #17269.

Related to #16117

Reason for this change

Currently, the only way to add additional security groups to the ECS Service outside of the constructor is to use escape hatches.

   const cfnService = service.node.defaultChild as CfnService;
    cfnService.addPropertyOverride(
      "NetworkConfiguration.AwsvpcConfiguration.SecurityGroups",
      [
        ...service.connections.securityGroups.map((sg) => sg.securityGroupId),
        someOtherSecurityGroupThatWasCreatedLater.securityGroupId,
      ],
    );

Description of changes

Use Lazy.list to set the service's security groups to its connections objects' security groups at synth-time.

This allows users to call service.connections.addSecurityGroup(someOtherSecurityGroupThatWasCreatedLater) at any point in their app and have someOtherSecurityGroupThatWasCreatedLater end up in the service's security groups at synth-time.

Description of how you validated changes

I added one test to fargate-service.test.ts

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 64377d7
• Result: SUCCEEDED
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