Support multiple plugins in one resource

[tbroden84]

Internal discussion about combine plugins into a single resource. Similar to how multiple firewall rules are handled. This will minimize the amount of resources needed if many plugins should be installed and minimize number of request made to the backend.

resource "cloudamqp_plugins" "plugins" {
  instance_id = cloudamqp_instance.instance.id

  plugin {
    name        = "rabbitmq_top"
    enabled	= true
  }

  plugin {
    name        = "rabbitmq_amqp1_0"
    enabled     = true
  }

  ...
}