(#18268) Specify service startup mode

This allows the service start mode to be specified on the command line as an
MSI parameter to the installer PUPPET_AGENT_STARTUP_MODE.

Valid choices are `Automatic` (default), `Manual` and `Disabled`. If you put in anything
besides these, it will default to `Automatic`.
Merge branch 'ticket/master/18268-specify_service_start_mode'

* ticket/master/18268-specify_service_start_mode:
  (#18268) Specify service startup mode