Fix S3 ListParts pagination infinite loop (#3679)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
The paginator for the S3 `ListParts` operation could loop forever:
https://github.com/awslabs/aws-sdk-rust/issues/1143. This is because
most paginated operations use an empty next token as an indication that
pagination is exhausted. `ListParts` instead sets the final next token
as `0` causing the pagination to loop back to the first page and loop
forever. Instead of an empty next token `ListParts` uses `IsTruncated =
false` to indicate that pagination has been exhausted.

## Description
<!--- Describe your changes in detail -->
* Added a new trait `isTruncatedPaginatorTrait`
* Add that trait to the S3 `ListPartsOutput` shape
* Use the presence of that trait to vary the logic setting the
`is_empty` value in the paginator.
  * If the trait is absent it looks for an empty next token as always 
* if the trait is present the value is set based on the value of the
response's `is_truncated` field


## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
* Added integration test confirming that pagination terminates when a
response contains `is_truncated = false`

**Note:** I'm still working on turning this into a model test rather
than an S3 specific integration test, but I wanted to get some feedback
on the actual fix while I'm figuring that out)

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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