This should be similar to kv.caches.do but having no TTL ... there are certain data sources which might be very slow and/or expensive, and we want to support an interface of https://keyv.do/:url like https://keyv.do/json.fyi/northwind.json

where the logic is:

1. Check KV if the URL as key exists
2. If key exists, return value
3. If key does not exist, fetch value from url
4. Save value to key with ctx.waitUntil with no TTL
5. Return value