introduce group option to apply descendants in groups

[mruoss]

Adds a group option to Bonny.Axn.register_descendant/3. This option controls in what group the descendant is applied. Groups are applied in ascending order. Resources within a group are applied simultaneously.

This option only needs to be specified if some resources must exist before others can be applied.

Example

  ns = %{
    "apiVersion" => "v1",
    "kind" => "namespace",
    "metadata" => %{"name" => "my_ns"}
  }
  pod = %{
    "apiVersion" => "v1",
    "kind" => "pod",
    "metadata" => %{"namespace" => "my_ns", "name" => "my_pod"},
    ...
  }

  axn
  |> Bonny.Axn.register_descendant(resource, ns, group: -1, omit_owner_ref: true)
  |> Bonny.Axn.register_descendant(resource, pod)

Solves #232.