Support for listing keys

[Erisa]

Ideally, we would use KV .list() when someone sends an authorized GET / but treat as a normal GET request otherwise.

Then return results like

[
  "/thing",
  "/thing2"
]

https://developers.cloudflare.com/workers/runtime-apis/kv/#listing-keys

For handling more than 1,000 keys maybe we can take the cursor and iterate? Not sure what the upper limit is like for KV ops in a request though...