Fix bug where Custom Auth Schemes were not respected (#3087)

## Motivation and Context
- Fixes #3034
## Description
Because AuthSchemeOptions were being registered at the operation level,
there was no way for them to be overridden by customer-provided runtime
plugins. This moves them into a separate plugin that is added at
Client/Default priority.

## Testing
- new unit test

## 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
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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