Accept AbortSignals in async methods

Methods like `.get()` should accept an AbortSignal to cancel the request.

AbortSignals are becoming more prevalent in platform and library APIs, and it's pretty easy to feed a signal from client-side code, into server requests handlers, and into libraries like firestore, which can send them to the underlying `fetch()` call, to cancel a database call.

Example using tRPC, which provides abort signals:

```ts
const appRouter = router({
getDocs: procedure.query(async ({signal}) => {
return db.collection('documents').get({signal});
}),
});
```