feat(config): Resolve configfiles using a custom PropertySource

[scottfrederick]

The Spring Cloud Config Server integration provided by kork-config includes the ability to specify account credentials (e.g. Kubernetes account kubeconfigfile and Google account json-path) as files that are stored in a Config Server backend like git, using a configserver: qualifier.

Previously code in services that accessed these file references in account properties needed to use a ConfigFileService bean provided by kork-config to resolve the file references, pulling the specified file from the Config Server backend if necessary.

This was starting to get unwieldy and intrusive in services, especially in cases where the code using the account properties wasn't implemented as Spring beans that could get a ConfigFileService bean injected into it. This PR adds a Spring PropertySource wrapper that automatically de-references the configserver: file reference when a property is resolved. The result is that services will always get a absolute local file reference.

ConfigFileService remains as a useful encapsulation of methods for dealing with local files.

[scottfrederick]

@ethanfrogers @ncknt If you're testing the config server integration in clouddriver using e.g. kubeconfigFile: configserver:kubeconfig.yml, this change should be transparent. I've tested with my k8s config, but any additional testing and review would be appreciated.