Add customization to ensure S3 `Expires` field is always a `DateTime` (#3730)

## 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 -->
At some point in the near future (after this customization is applied to
all existing SDKs) the S3 model will change the type of `Expires`
members to `String` from the current `Timestamp`. This change would
break backwards compatibility for us.

## Description
<!--- Describe your changes in detail -->
Add customization to ensure S3 `Expires` field is always a `Timestamp`
and ass a new synthetic member `ExpiresString` that persists the
un-parsed data from the `Expires` header.

## 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 tests to ensure that the model is pre-processed correctly. Added
integration tests for S3. Considered making this more generic codegen
tests, but since this customization will almost certainly only ever
apply to S3 and I wanted to ensure that it was properly applied to the
generated S3 SDK I opted for this route.

## 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 AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._