Support passing credentials to Dyno as an alternative to passing Access Keys directly

[drboyer]

We should add support for a credentials option in the Dyno constructor options so that instead of having to specify the actual AWS Access Key and Secret Key (if you don't want to use the defaults), you can use any of the various Credential Providers/subclasses (see here). This allows you to do useful things like use TemporaryCredentials from an assumed role that will automatically refresh as needed.

This issue actually blocks implementation of Kine #42