aws-step-function-tasks: Allow removing launchType from RunEcsEc2Task

capacity providers allow using ECS with dynamic capacity. https://aws.amazon.com/tw/about-aws/whats-new/2019/12/amazon-ecs-capacity-providers-now-available/ , yet CloudFormation and CDK do not yet support this feature https://github.com/aws/aws-cdk/issues/5471.

I propose as a stop gap solution to allow users to submit EC2 tasks without `launchType`:
https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_RunTask.html#ECS-RunTask-request-launchType

if `launchType` is provided, there is no way to get the task to use the capacity providers that are created outside CDK.

### Use Case

I want to create capacity providers using boto3 and use them when launching EC2 tasks using step functions.

### Proposed Solution

Provider a way to make `launchType` optional here:
https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/aws-stepfunctions-tasks/lib/ecs/run-ecs-ec2-task.ts#L62

Or to change it after construction.

### Other

* [x] :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 packages/@aws-cdk/aws-stepfunctions-tasks/lib/ecs/run-ecs-ec2-task.ts at the RunEcsEc2Task definition and trace how launchType is configured. Done means EC2 tasks can be submitted without launchType for externally created capacity providers while existing explicit launchType behavior remains supported.

Written by the indexing model from the issue text.