Zhessler add dsql auth token generator (#3927)

## Description
<!--- Describe your changes in detail -->
This adds an auth token generator that works like the existing RDS auth
token generator, but for a new service: DSQL.

To generate the DSQL SDK, run this command from the root of the project:

```
./gradlew :aws:sdk:assemble -Paws.services=+sts,+sso,+ssooidc,+dsql
```

Then, to run the auth token generator tests:

```sh
cd aws/sdk/build/aws-sdk/sdk/dsql
cargo test signing_works --all-features
```

## 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. -->
tests are included in the inlineable

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

----

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copy, and redistribute this contribution, under the terms of your
choice._

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Co-authored-by: ysaito1001 <awsaito@amazon.com>