[ECS] [ARN and ID]: Get Account Default Opt-In Status

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**Tell us about your request**
As part of #126 the `PutAccountSettingDefault` API call was created. I would like a call to `ListAccountSettingDefault`.

**Which service(s) is this request for?**
ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I currently have no way to verify the change and/or check the existing settings. I don't believe this is possible even with the root user at this time.

**Are you currently working around this issue?**
No - we are using the Put call to override whatever is existing, but no way to verify in advance.

**Additional context**
N/A

**Attachments**
N/A

No implementation files, tests, or entry points are named. Start by reviewing the requested ECS ListAccountSettingDefault call alongside PutAccountSettingDefault; done means users can retrieve existing default account settings and verify changes.

Written by the indexing model from the issue text.