add the before_start_hook

[rgl]

this is a PoC for #198.

I'm looking for suggestions about the name of the configuration property, currently, its called before_start_hook.

if you agree with this change, I'll later will be adding the missing documentation.

the before_start_hook configuration setting contains the command that will be executed after the vm is configured, but before its started.

the command receives the current vm configuration as a json in its stdin

the hook command is expected to return a json with the modified configuration, typically, this document should only contain the properties that you want to actually modify.

the following example is modifying the ide0 and ide1 media from cdrom to disk, something that is useful when we have a disk image instead of an iso:

source "proxmox-iso" "macos-amd64" {
  # ...
  before_start_hook = [
    "python3",
    "-c",
    <<-EOF
    import json, sys
    config = json.load(sys.stdin)
    print(json.dumps({
      "ide0": config["ide0"].replace("media=cdrom", "media=disk"),
      "ide2": config["ide2"].replace("media=cdrom", "media=disk"),
    }))
    EOF
  ]

Closes #198

[rhpijnacker-philips]

It seems to me this is a bit of a kludge.
It means that we are not currently able to express the desired configuration with the existing parameters.

[rgl]

@rhpijnacker-philips yes, like described at #198, this is an escape hatch, to use in those cases where there is no other way.

[medioxor]

i am desperate for this! very excited as i'd like to automate mac deployment