[3161] Implement Debug for DateTime (#3619)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
According to #3161 , the Debug output was in a marginally legible
format.

<!--- If it fixes an open issue, please link to the issue here -->
#3161 

## Description
<!--- Describe your changes in detail -->
Since there's no expected difference between `Debug` and `Display` for
date_time, the `fmt::Debug` for `date_time` calls the already
implemented `Display`.

## Testing
<!--- Please describe in detail how you tested your changes -->
I added unit tests, and ran `cargo test` within the `rust-runtime`
directory.
<!--- Include details of your testing environment, and the tests you ran
to -->
Aarch64 mac terminal - `$ cd rust-runtime && cargo test`
<!--- see how your change affects other areas of the code, etc. -->
A single test failed:
`client::waiters::backoff::tests::backoff_with_seeded_jitter` at first.
After rebasing on upstream, the failure went away. Thanks for the fix.



## 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

----

_By submitting this pull request, I confirm that you can use, modify,
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choice._

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Co-authored-by: Mike Nissenbaum <mnissenb@amazon.com>