Guided ECS Exec

**Is your feature request related to a problem? Please describe.**
We have a number of environments that we need to support ECS Exec in. We have written a python script to navigate through the environment to help fill in the ECS Exec command. It would be great if this could be something built into the normal aws cli.

**Describe the solution you'd like**
* `aws ecs execute-command-guided`
* List ECS Clusters if more than 1 in the environment to allow you to pick which one
* List Services with ECS Exec enabled
* List containers in the task that are running
* Iterates through the tasks to find one that is available to be connected to. This is to handle someone else already being connected to a task.

**Describe alternatives you've considered**
We wrote our own own wrapper python script.

**Additional context**
This would make it a lot simpler for end users to start using ECS Exec without going through finding all the details required.

Happy to provide a PR if I can be pointed to where it should go.