Refactor the cache overrides implementation

[conico974]

We should refactor the cache overrides implementation to better reflect current and upcoming Next features.

• Separate incremental cache entry into the value (as a ReadableStream) and what we'll call metadata (next metadata and things like revalidatedAt)
• Better bypassTagCache by allowing the incremental cache to return tag information directly
• Rework the CDN invalidation overrides (not sure yet how)
• Implement a Cache function/worker, that could send all that in a single cached response (with appropriate Auth if necessary)

The idea by separating value and metadata is to be able to stream the value ASAP, especially useful for Composable cache.
We ideally also want to do a single trip for the cache