Add support for contract management API

[bkircher]

Start building support for a meaningful subset of contract management API (https://api.gridscale.io/management/v1/).

Interesting sub-set would be to get out current billing information for a single contract.

[bkircher]

Note that contract management API is an internal name for that part of the API and... really misleading because it is not responsible at all for contracts 🌮 🌮 🌮

[fabiante]

This is https://my.gridscale.io/Contract-Management-v2 now.

To me it seems sensible to not include contract management in this client but instead have a new client for that. One reason is that the authentication is different from the regular API. Another reason is that confg options for the contract API would also be different given that it uses another URL.

Of course it would also be an option to add that to this package, then that would require refactoring or the introduction of a whole new set of types. Maybe adding that as a dedicated sub-package is best.