Make it easier to use write hooks that involve passing references from preceding write operations

[achou11]

Extracted from #20

The most notable example of this is when creating/updating observations that have attachments. Right now, the end-user has to work with the relevant write hook to create the attachment(s) prior to creating the observation e.g.

const createBlob = useCreateBlob(...)

const createObservation = useCreateDoc({ docType: 'observation', ... })

// Example of a single attachment
function onPress() {
  createBlob.mutate(..., {
    onSuccess: (blob) => {
      // Create observation that references blob after its creation
      createObservation.mutate({ value: { attachments: [{ ...blob, driveDiscoveryId: blob.driveId }] }, ...})
    }
  })
}

// Example of multiple attachments
function onPress() {
  const blobs = await Promise.all([
    // Necessary if using useCreateBlob() because we don't expose a Promise-based method like React Query's `mutateAsync()`
    new Promise((res , rej) => createBlob.mutate(..., { onSuccess: res, onError: rej})),
    new Promise((res , rej) => createBlob.mutate(..., { onSuccess: res, onError: rej})),    
  ])

  // Create observation that has multiple attachments
  createObservation.mutate({ value: { attachments: blobs.map((b) => ({ ...b, driveDiscoveryId: blob.driveId })), ... } })
}

The idea would be to have something similar to what the original issue outlined such that the end-user code could look more like this:

createObservation.mutate({ value: ..., attachmentsToAdd: [...] })

As mentioned in #26, I didn't figure out a satisfactory way of implementing this and decided to leave that as a follow-up, since it's mostly for end-usage convenience.