[ECS] [request]: Scheduled Tasks support for ECS-A (ECS-Anywhere)

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**Tell us about your request**
Scheduled Tasks support for ECS-A (ECS-Anywhere)

**Which service(s) is this request for?**
This could be ECS Anywhere

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I can set the EXTERNAL launch type in Task and Service but not in scheduled task.

Can't find the EXTERNAL launch type in console or CLI

**Are you currently working around this issue?**
How are you currently solving this problem?

**Additional context**
Anything else we should know?
When I execute the CLI and use EXTERNAL as launch type the CLI will return the error below
`An error occurred (ValidationException) when calling the PutTargets operation: 1 validation error detected: Value 'EXTERNAL' at 'targets.1.member.ecsParameters.launchType' failed to satisfy constraint: Member must satisfy enum value set: [FARGATE, EC2]`

**Attachments**
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No repository file or test is identified in the issue. Start by reviewing the ECS scheduled-task CLI flow and the PutTargets validation error for the EXTERNAL launch type. Done means scheduled tasks support ECS-Anywhere with EXTERNAL launch type in the console or CLI.

Written by the indexing model from the issue text.