Add network policy support to go-cfclient

[sneal]

It would be useful to be able to make network policy API calls using the go-cfclient. The resources would need to be added and a new client. I don't know if this should go into it's own package or even it's own separate library, but it seems as if adding it here would be easiest for consumers.

Here's a sample POC using the CF network policy API

[vipinvkmenon]

It looks like we will need to bring this support in, as it is something that people are asking for from the terraform provider and it makes sense to bring it generally on the client so that it caters to a larger audience

[vipinvkmenon]

Would this work: https://github.com/cloudfoundry/policy_client