An async version of RequestPermission?

### Problem

Currently, the method `RequestPermission` of struct `acp.Client` has to return the user choice, or it must be synchronous. This makes it very hard to implement an UI for this (not CLI), because the UI must not block so that users can select their choice. And I'm not even sure if the SDK allows that method to run in another goroutine.

### Solution

I think there should be a `AsyncRequestPermission` that
- has request ID in its params
- Doesn't return user choice

And a function `SendPermissionChoice` that send user choice with request ID to the ACP server

@ThomasK33