Adding impl Display for enum (#3391)

## Motivation and Context
This change address the
[#3335](https://github.com/smithy-lang/smithy-rs/issues/3336)

## Description
Added Display trait to the Enum. 

## Testing
Updated the
[ClientEnumGeneratorTest.kt](https://github.com/smithy-lang/smithy-rs/blob/main/codegen-client/src/test/kotlin/software/amazon/smithy/rust/codegen/client/smithy/generators/ClientEnumGeneratorTest.kt

)
which test the ```impl Display``` for the enums.

## 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
----

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Co-authored-by: Prince Kumar Maurya <princekumarmaurya06@gmail.com>