[maven-aws] Adds support for AWS S3 backed maven repositories (resolutio...

...n not publishing).

[maven-aws] Adds skeleton S3 transport classes and uses transport scheme to determine credentials type.

[maven-aws] Wiring in AWS sdk and implementing S3 resource accessor

[maven-aws] Code review cleanup

[maven-aws] Sets default credentials to null when specific type has been specified via DSL

[maven-aws] Validates the PasswordCredentials type
alternativeCredentials field never used in production, we should get rid of it. We could instantiate the default PasswordCredentials lazily when calling credentials(Action<? super PasswordCredentials>)