Allow batch lookups

[ruurtjan]

Many HTTP APIs have a batch endpoint. This allows multiple values to be requested with a single HTTP call.

This doesn't work well with ZIO-cache right now, as there is no way to look up multiple values at once.

HTTP endpoints are prime targets for caching, since network overhead is usually significant. So I think support for this use case would be a great addition to ZIO-cache.

I'm not sure what the best interface for this would be, but ideally, it would:

• Use cached values for keys already present in the cache;
• Call the user-defined batch function with the remaining keys (if any);
• Add new entries to the cache;
• Enforce at the type level that the user-defined batch function returns a value for every key.

[regiskuckaertz]

I actually had the same idea than you and thought that half of your use case is already addressed by zio-query, but it lacks the caching part!

zio/zio-query#378