Defines a tool

Example

tool "echo", "Echoes back the input", EchoController, :echo,
  title: "Echo",
  hints: [:read_only, :non_destructive, :idempotent, :closed_world] do
  input_field("message", "The message to echo", :string, required: true)
  output_field("message", "The echoed message", :string)
end