Allow adding cache tags that depend on the response of fetch or unstable_cache

[romeovs]

Goals

In some cases it might be useful to provide cache tags to a cache entry, after the result has been received.

For instance: I might fetch a product from the api:

await unstable_cache(() => api.products.get({ id }), ["product", id], {
  tags: [`product:${id}`],
})
// { type: "product", id: 42, ... }

however, I might want to include related products in the response:

await unstable_cache(() => api.products.get({ id }, { related: true }), ["product", id], {
  tags: [`product:${id}`],
})
// { type: "product", id: 42, related: [{ type: "product", id: 85 }, ...], ... }

It might be useful to be able to tag the response with the ids of those products, even though we only know those after we've received the response. That way, when a related product changes, it can correctly invalidate this response.

Something similar might also be useful for revalidate.

Non-Goals

1. listing current cache tags
2. changing the API in a non-backwards-compatible way

Background

One example where this is used is RTK Query, where providesTags is allowed to be a function that takes the response.

In next something similar could look like:

await unstable_cache(() => api.products.get({ id }, { related: true }), ["product", id], {
  tags(product) {
    return [`product:${id}`, ...product.related.map(p => `product:${p.id}`)]
  }
})
// { type: "product", id: 42, related: [{ type: "product", id: 85 }, ...], ... }

Proposal

Allow passing an (async) function to tags that gets passed the response and returns a list of tags.