Auto-population for KV

[james-elicx]

We need to migrate the KV adapter to populate the KV namespace instead of relying on static assets as a fallback. This also unlocks being able to use TTL for KV properly.

A few more things we would tackle while updating the cache handlers:

• debug in R2 should be enable when process.env.NEXT_PRIVATE_DEBUG_CACHE === 1 (see KV)
• the handler and the the populate cache should share constants instead of repeating magic strings in different files
• we can now assume that process.env and the bindings are available in the ctor and do not need to be lazily retrieved