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

Extracted from https://github.com/digidem/comapeo-core-react/issues/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() {
  Promise.all([
    createBlob.mutateAsync(...),
    createBlob.mutateAsync(...),
  ]).then(attachments => {
    // 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 https://github.com/digidem/comapeo-core-react/pull/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.

No files or tests are identified in the issue. Start by reviewing the existing write hooks, the extracted issue, and pull request #26 to understand the rejected approach; done should let callers pass attachments to the observation mutation without manually chaining blob creation operations.