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.  

 
I changed DiskType.ValidDiskSize to Optional in the rebased and squashed commit below. 

I realize now it may have been better to not squash the commit so the minor changes would have been more noticeable.