[Model Server] Support custom workspace URI schemes #180

Umbrella issue for changes in the Model Server repos emfcloud-modelserver, modelserver-node, emfcloud-modelserver-theia, emfcloud-modelserver-client

• Allow to handle custom workspace URI schemes
• Handle workspace relative URIs on Java Model Server side (modelserver-node redirects calls to Java server by default)
• Optional:
  • Helpers to build URIs from file paths, relative paths on client side (Theia clients)

URI examples using workspace schemes:
workspace://ws-uuid001/my/relative/path.ext
custom://<workspace-id>/<project-id>/<model-type>