smithy-rs

## Motivation and Context
- aws-sdk-rust#901
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->

## Description
Identity resolvers aren't a list—they are a map keyed on AuthSchemeId.
This updates the internal representation to match.

## Testing
<!--- Please describe in detail how you tested your changes -->
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to -->
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## 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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choice._

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Co-authored-by: John DiSanti <jdisanti@amazon.com>