Support extended memory when creating VMs

[LukasWallrich]

Thanks for this very helpful tool. I am not sure if I am missing a way to do this, but I would love to be able to create a VM using extended memory

When I just try to request more memory than the usual maximum per CPU, I (expectedly) get an error - but I have no idea how to pass the equivalent of gloud's custom-extensions parameter ... could that be supported?

[Tiny note: in ?gce_vm, the link in the documentation for minCpuPlatform does not work.]

[MarkEdmondson1234]

I wasn't aware of extended memory yet ;) it should be relatively simple to add to the json api call. May also be worthwhile allowing one to pass any additional api body to make it more future proof for other features.