ACP: support an ask_user / ask_question style extension method

## Summary

Please consider adding ACP support for an `ask_user` / `ask_question` style extension method so custom ACP clients can surface clarifying questions and return structured answers to Copilot CLI.

Today ACP support in Copilot CLI includes `session/request_permission`, but that only covers allow/deny tool approvals.

## Why this would help

Custom ACP frontends can already render streaming messages, plans, tool calls, and permission prompts. The missing piece is a first-class way for the agent to ask the human a question and wait for a reply.

That makes it hard to support workflows that depend on interactive clarification, even though GitHub Copilot itself supports this kind of host-side interaction in some environments.

A small extension method would make custom ACP frontends much more capable without requiring each client to invent incompatible workarounds.

## Suggestion

A Copilot-specific ACP extension would already be helpful, for example something like:

- request method: `copilot/ask_user` (or similar)
- params:
- `sessionId`
- `question`
- optional `choices: string[]`
- optional `allowFreeform` / `allow_freeform: boolean`
- result:
- `{ outcome: "answered", answer: string }`
- or `{ outcome: "cancelled" }`

Cursor has `cursor/ask_question` Source: https://cursor.com/docs/cli/acp#cursor-extension-methods