fix: support signal on getDocument(s) to cancel requests

[stipsan]

Allows cancelling getDocument and getDocuments with an AbortController when using the promise API:

import {createClient} from '@sanity/client'
import {useEffect} from 'react'

const client = createClient({ /* ... */ })

const [document, setDocument] = useState(null)
useEffect(() => {
  const controller = new AbortController()
  client.getDocument('abc123', {signal: controller.signal}).then(setDocument)
  return () => controller.abort()
}, [])

The ObservableSanityClient doesn't need this, as it automatically cancels inflight requests when the observer no longer has any subscribers.

[rdunk]

LGTM!