Add RDS URL signer (#3867)

## 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 -->
[aws-sdk-rust/951](https://github.com/awslabs/aws-sdk-rust/issues/951)

## Description
<!--- Describe your changes in detail -->
Adds a struct for generating signed URLs for logging in to RDS. See
[this
doc](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/UsingWithRDS.IAMDBAuth.Connecting.html)
for more info.

## Testing
<!--- Please describe in detail how you tested your changes -->
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to -->
<!--- see how your change affects other areas of the code, etc. -->
I wrote a test.

## Checklist
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rather than leaving it unchecked -->
- [ ] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [ ] 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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