Make it easy to turn ecs TaskDefinitions into json strings

in the package @aws-cdk/aws-ecs on the class TaskDefinition please add a toJSON method that
returns the complete task definition as a JSON

### Use Case

This would be used to have a template taskdef.json for codedeploy.

### Proposed Solution

Add toJSON method on TaskDefinition

### Other

* [ ] :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

Start in the @aws-cdk/aws-ecs package at the TaskDefinition class and inspect how the task definition is currently represented. Add the requested toJSON entry point so it returns the complete task definition as JSON, then verify that the result can serve as the template taskdef.json needed for CodeDeploy.

Written by the indexing model from the issue text.