store identity resolvers in a map (#3363)

## Motivation and Context
- aws-sdk-rust#901
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## Description
Identity resolvers aren't a list—they are a map keyed on AuthSchemeId.
This updates the internal representation to match.

## Testing
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## Checklist
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- [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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Co-authored-by: John DiSanti <jdisanti@amazon.com>