Add serde support to date time (#2646)

## Motivation and Context
This is a child PR of https://github.com/awslabs/smithy-rs/pull/2616

The changes that this PR introduces is same as the ones that were merged
to `unstable-serde-support` branch before.

Initially, we tried to make commit to unstable-serde-support branch and
merge changes one by one in small PRs. However, in order to make it up
to date with the main branch, we would need to go through a large PR of
over 700 files.

Thus, I decided to create individual PRs that commits directly to `main`
branch.

## Description
- Implements `serde` support to `DateTime`

## Testing
- Test checks whether the serialized/de-serialized data matches with the
expected value

## Checklist
NA

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