Add AbortSignal support to unary calls

With callback client I can do

```ts
const call = client.method(request, meta, cb)
```

While Promise client doesn't give that `call` but I need it to `call.cancel()` in the Timeout

---

BTW another question - Am I have to do that? I want to cancel my call if it exceeded the timeout, but in my `meta` I have `deadline`. Will grpc-web cancel my call for me or I have to do it manually? If it does, will promice reject with timeout error?

Thank you!