Support `stringArray` type in endpoints params (#3742)

## 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 -->

## Description
<!--- Describe your changes in detail -->
Updated our endpoint rule generation to support the new `stringArray`
parameter type

## 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. -->
Updated the existing `EndpointsDecoratorTest` with tests for
`stringArray`

## 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: Zelda Hessler <zhessler@amazon.com>