Support .promise() pattern for RDS.Signer.getAuthToken

[willfarrell]

Is your feature request related to a problem? Please describe.
Currently I need to do the following:

const signer = new RDS.Signer(config)
    return new Promise((resolve, reject) => {
      signer.getAuthToken({}, (err, token) => {
        if (err) {
          reject(err)
        }
        resolve(token)
      })
    })
    ```


**Describe the solution you'd like**
I would like to do:
```javascript
const signer = new RDS.Signer(config)
    return  signer.getAuthToken({}).promise()

Describe alternatives you've considered
See top of issue for current workaround. I looked into using v3 but RDS.Signer is not supported.

Additional context
This API is inconsistent with how other API are handled.

[ajredniwja]

Hey @willfarrell thanks for opening this feature request.

Note: The version 3.x of the AWS SDK for JavaScript is generally available.
For more information see the Developer Guide
or API Reference.

[willfarrell]

I would consider using the v3 SDK, however RDS.Signer isn't supported. aws/aws-sdk-js-v3#1823

[ajredniwja]

Thanks for mentioning that issue, It would more likely be implemented in V3 first as that is the SDK that is being actively being worked on.

[clafollett]

I know I could definitely use the functionality of RDS.Signer in v3!!!

[github-actions]

Greetings! We’re closing this issue because it has been open a long time and hasn’t been updated in a while and may not be getting the attention it deserves. We encourage you to check if this is still an issue in the latest release and if you find that this is still a problem, please feel free to comment or open a new issue.