Add support for file based service bindings

[beyhan]

There is a limit regarding the size of environment variables described in this CF issue, that is why in RFC-0030 the CF community decided to introduce a different option for service binding provisioning in the application container which is file based. The RFC describes two ways to provide the bindings in the app container one based on the K8s specification and another one providing the VCAP_SERVICES content as file, which location is specified with the VCAP_SERVICES_FILE_PATH environment variable. The feature is already available in CF and more details can be found in cloudfoundry/community#901

The go-cfenv client should add support to read service binding information from a file defined by the VCAP_SERVICES_FILE_PATH environment variable. With this users would be able to switch between VCAP_SERVICES environment variable or the VCAP_SERVICES_FILE_PATH activated via the app-feature file-based-vcap-services without the need to do any application changes other then using the go-cfenv version which supports this. I don't think that adding support for the K8s specification makes sense because a translation in the go-cfenv library from K8s specification to VCAP_SERVICES isn't possible as discussed here.