Add upsert entity hook

Often you'll want to modify an entity if it exists, or create it if it doesn't. For example, say you land on a page for a new TV show. You might want to add the TV show to your "Watch later" list and add a note on why you saved it. To do this, the code looks like this:

const maybeSavedShow = useQueryEntity(...)
const updateSavedShow = useUpdateEntity(..., maybeSavedShow?.id ?? '')
const createSavedShow = useCreateEntity(...)

const onClick = maybeSavedShow ? updateSavedShow : createSavedShow

<button onClick={onClick}>Add to watchlist</button>

Start by locating the existing useQueryEntity, useUpdateEntity, and useCreateEntity hooks referenced in the example. Define the new upsert hook around the shown existing-versus-new entity flow, and verify that it updates when the entity exists and creates it otherwise.