gocbcore should support communicating with the Eventing/GSI and Backup Services

What's the issue?
The tooling in the 'backup' repository currently relies on a slimmed down REST client which implements a fair amount of functionality which already exists in the SDK. We'd like to move over to using the SDK where possible, however, there's a few missing features that we'd need to make this possible.

1) We'd need to be able to communicate with the:
1a) Eventing service
1b) Indexing service
1c) Backup service
2) We'd like to be able to access the underlying KV endpoints (which currently aren't exposed via the 'Agent')
2a) We'd need to be able to preserve a scheme prefix for the KV endpoints to allow the SDK to return an SSL endpoint if required (the 'backup' tooling then doesn't need to worry about this)