Add support for Spot VMs

[igrikus]

It would be great to have the ability to provision Spot VMs for specific runner pools to save costs.

I see this as an additional boolean parameter within the extra_specs for a pool

Here is the official documentation for creating Spot VMs:
Create and use Spot VMs

Things to consider:

Spot VMs can be terminated by GCP at any time. The Garm provider should handle this as an expected, normal event for this type of instance.