Support configuration of an AWS credential callback

The Java driver added support for a callback to supply an AWS Credential, in scope of JAVA-4310. A user on JAVA-4118 pointed out that the callback can't be configured in Kafka Connect. We should consider allowing configuration of that callback in Kafka.

For example, we could introduce a new configuration property that allows an application to specify the name of the class that implements Supplier<AwsCredential>, require it to have a no-arg constructor, and then create and use in the MongoClient created for the task.