feat: ability to run custom callback on timeout for Absinthe.Middleware.Async middleware

[freevova]

Async middleware sends an exit signal that kills the request and causes a 500 on timeout, as it uses
Task.await/2 under the hood. This PR adds the ability to pass on_timeout/1 callback function to
the middleware and trigger it on timeout.

By default, it stops the current process with exit/1, as before.

field :time_consuming, :thing do
  resolve fn _, _, _ ->
    async(
      fn -> {:ok, long_time_consuming_function()} end,
      timeout: 7_000,
      on_timeout: fn _timeout ->
        {:error, "Failed to get a result"}
      end
    )
  end
end

The only potential issue may be that in case someone uses try/catch like this:

    try do
      ...
    catch
      :exit, {:timeout, {Task, :await, _}} ->
        IO.inspect("catched error")
    end

it won't work, as now it exits with exit({:timeout, {Absinthe.Middleware.Async, :call, [task, timeout]}})
If we need to handle it, the only way that I see is to pattern match on options and call:

• Task.yield/1 + Task.shutdown/1 when the on_timeout option is present
• Task.await/2 otherwise

After I had already implemented the logic, I noticed that such a question had already been raised.