fix: type spec on unique field

[andrew-combs]

I noticed that when implementing the Oban.Job.new/2 callback, specifying a list of unique fields caused a "No local return" warning:

defmodule MyApp do
  use Oban.Worker

  @impl true
  def new(args, _opts) do
    # Function new/2 has no local return.ElixirLS Dialyzer
    Oban.Job.new(args, worker: __MODULE__, unique: [fields: [:args], keys: [:id]])
  end

end

This appears related to the Oban.Job.unique_field type being a list type, which is then wrapped again in the @type unique_option :: {:fields, [unique_field()]} | ... type.

For consistency, I removed the list spec in the unique_field type rather than the unique_option type.

[sorentwo]

Thanks for the fix!