implement `Ord` and `PartialOrd` for `DateTime` (#2653)

## Motivation and Context

This change will allow easy sorting or comparing anything that contains
a DateTime. My example is wanting to sort a list of S3 objects by last
modified.

This PR fixes #2406

## Description
It's a pretty small PR, it implements the `Ord` trait for `DateTime` by
comparing the `seconds` property of `self` and `other`, if they are
equal it compares the `subsec_nanos` properties instead.

The `PartialOrd` trait is implemented by calling the `Ord` trait.

## Testing
I added a unit test that compares a number of `DateTime` values with
different combinations of positive/zero/negative
`seconds`/`subsec_nanos`.

## Checklist
- [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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