datasource/cognito: add cognito provider

[calebdoxsey]

Summary

Add a Cognito directory provider.

By default the AWS SDK will search for credentials from a variety of pre-defined sources on the machine where the code is run. Credentials can also be specified manually.

A user pool can be specified or if none is provided we will list all the users in all the user pools.

Related issues

• https://github.com/pomerium/internal/issues/1691

Checklist

• [x] reference any related issues
• [ ] updated docs
• [ ] updated unit tests
• [ ] updated UPGRADING.md
• [x] add appropriate tag (improvement / bug / etc)
• [x] ready for review