Add support for C3 instances with IDPF network interfaces

[JoelSpeed]

/kind feature

Describe the solution you'd like
[A clear and concise description of what you want to happen.]

Google C3 instances support a different kind of advanced network interface, called [IDPF|https://cloud.google.com/compute/docs/networking/using-idpf]. I would like to use this interface type with these instances via CAPG, and so, need to expose via the MachineTemplate a way to configure the type of the network interface.

At present, CAPG omits the NicType field from the compute.NetworkInterface, but, the default is GVNIC (Standard gcp network interface), this new interface is just IDPF.

We should add an enum value to the spec that allows this to be configured by end users.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

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