Support http:// and file:/// URIs in microgateway settings

[pointlander]

Currently, only flogo resources are supported in the microgateway URI setting:
https://github.com/project-flogo/microgateway/blob/master/examples/json/basic-gateway/flogo.json#L75

'http://' and 'file:///' should be added as supported URI types. If a URI doesn't have a 'http://' or 'file:///' prefix then it is assumed to be a flogo resource. When the URI is 'http://' the microgateway definition will be fetched using http, and when the URI is 'file:///' the microgateway will be loaded from the local file system. Some form of caching should be used for the microgateway definitions.