(aws-ecs): Provide an enableExecuteCommand method

When the `enableExecuteCommand` property was added, the `enableExecuteCommand()` method was made private. The logic that handles this property should have also been exposed as a method so that this feature could be enabled subsequent to construction of the construct.

### Use Case

For use cases where we do not have direct access to construction of the construct, such as in the case of `ApplicationLoadBalancedFargateService`.

### Proposed Solution

Move all logic related handling of the `enableExecuteCommand` configuration property into the `enableExecuteCommand()` method and make that method public.

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

Locate the AWS ECS construct implementation that handles the enableExecuteCommand property and the ApplicationLoadBalancedFargateService entry point. Compare the existing property handling with the private enableExecuteCommand() method, then verify that the behavior can also be invoked after construction while preserving current configuration behavior.

Written by the indexing model from the issue text.