Add coverage of DiskType Api

I figured before adding support for SSDs it makes sense to add coverage of the DiskTypes Api. 

DiskTypes Api Documentation: https://cloud.google.com/compute/docs/reference/latest/diskTypes 

The implementation is very similar to MachineTypes Api. Refactoring redundancies may be desirable in the future. 

Also like MachineTypes this only covers the "get" and "list" operations and does not yet cover the "aggregatedList" operation.  

 
BTW, it should be as easy as adding the pageToken (or marker) parameter in the `ListOptions`, and all the redundant methods would be really redundant and we should be able to remove them.