feat: association_type resource

[thijssimonis]

The possibility to create/update Akeneo association types.

resource "akeneo_association_type" "bundle" {
  code          = "bundle"
  is_quantified = true
  is_two_way    = false
  labels = {
    en_US = "Bundle"
  }
}

[0xfrej]

In future, resources should probably prohibit mutations of it's identifiers (i.e. category code). I thought it already did on other resources, but I was mistaken.

Also the amount of non-removable resources is piling up. It might be a good idea to write some support plugin for Akeneo, but I don't have much time for it at the moment.

LGTM