Implement DummyCheckTask and move dummy into embedded in-memory checks

[dnsmichi]

This replaces the previous "dummy" CheckCommand, and the user won't
notice it. Provided performance data will be parsed the same way.

This saves a shell fork and check_dummy execution.

We're relying on this when creating cluster checks with Icinga 2 DSL
and more. If one does not have the plugins installed, this then also works.

fixes #5740

[dnsmichi]

Tests

object Host "dummy-host" {

  check_command = "dummy"

  vars.dummy_state = {{ Math.random() * 10 }}
  vars.dummy_text = {{
    var state = macro("$dummy_state$")
    if (state > 1) {
      return "Very critical indeed."
    } else {
      return "I don't care."
    }
  }}

  check_interval = 1s
  retry_interval = 1s
}

Via REST API, command is null, and not check_dummy.

[dnsmichi]

Oh, and this works in Windows too 👌