Add support for using stdin and pipes

### Describe the feature

Per https://github.com/aws/aws-cli/pull/3209#issue-306823377:

> I'd like to use `aws` in a pipeline, as per [this question](https://superuser.com/q/1306071/707676).
> It seems that `aws` will read the indicated file twice, using the second dataset for operation - in a pipeline, the second dataset will be empty...
>
> I've implemented something basic that works - use the `stdin://` prefix/schema.

### Use Case

See above

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CLI version used

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### Environment details (OS name and version, etc.)

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