Move gitlabnet client to client package

fixes: #451 (closed)

This MR:

1. moves both the internal/gitlabnet client methods and the http client in internal/config package to client
2. changes the client constructors to be purely dependency injected so as to not be dependent on a config struct
3. moves internal/gitlabnet tests
4. keeps the GetClient method the same so we don't have to change too much code