Add callback to rest context

[quinchs]

Summary

This PR adds the InteractionResponseCallback property to RestInteractionContext and introduces the interface IRestInteractionContext. This allows the process of writing the respond function much simpler, ex:

async Task WriteCallback(string payload)
{
    await response.OutputStream.WriteAsync(payload);
}

var context = new RestInteractionContext(client, interaction, WriteCallback);
await interactionService.ExecuteCommandAsync(context, services);
response.StatusCode = 200;
response.Close();

If no callback is passed into the ctor of RestInteractionContext then the default global callback is used, the callbacks are mutually exclusive.

[quinchs]

cc: @Cenngo