Add validation and configuration for supported resource types for registered apps

[trisberg]

Different server implementations support different types of resources, some support Maven and some only support Docker resources. We should have a way to configure the server with supported types and validate when resources are registered. That way we could provide an error when someone registers an unsupported resource type.

https://github.com/spring-cloud/spring-cloud-dataflow-server-kubernetes/issues/157

[markfisher]

Maybe we can just add a supportsScheme(String scheme) method to the DelegatingResourceLoader.

One thought on this though... we can validate the "type", but it's still completely possible that a user registers a bogus URI. We probably cannot eagerly validate anything more than the scheme, unless we want to provide a boolean flag for eager validation.

[sabbyanandan]

The core validation routines are applied via #2345 and friends. They include for both maven and docker resource types.