podman: further isolate ignition code

In podman machine, the machine package tends to be the "dumping ground" for a lot of code, including the ignition implementation. We want to move the ignition implementation from the machine package into its own package located at pkg/machine/ignition/. 

There are two parts to this:

• isolate the ignition code into its new package

• • This involves preventing any circular dependencies.
• abstract commonalities
  • There is some common code that is re-implemented amongst the virt providers (hyperv, applehv, qemu). We want to abstract this out and implement additional methods for the IgnitionBuilder type to create a common code path. There are also large blocks of strings, such as the different Unit files, which should be broken down into composable pieces where possible.

This work should not be merged upstream until after we branch 4.8.