Add EnumSection to allow decorators to modify enum member attributes (#4039)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
Allows users to use decorators to add additional attributes to members
of an enum.

<!--- If it fixes an open issue, please link to the issue here -->
N/A

## Description
<!--- Describe your changes in detail -->
Adds `EnumSection` with named `AdditionalMemberAttributes` to allow
decorators to modify enum codegen.

## Testing
<!--- Please describe in detail how you tested your changes -->
Could not find existing unit tests for customizations; please advise if
you would like them to be added here. Ran `./gradlew` to ensure current
tests pass.
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [X] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.


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choice._

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Co-authored-by: Landon James <lnj@amazon.com>
Co-authored-by: ysaito1001 <awsaito@amazon.com>