Add key listing functionality for workers kv subcommands

[gabbifish]

This PR closes #345. A clarification: this does not list all KV pairs in a Workers KV instance, but instead, lists all keys and (if present) their expiration timestamps.

Usage without prefix filter:

$ /wrangler kv list-keys 06779da6940b431db6e566b4846d64db
[{"name":"key1"},{"name":"key2"},{"name":"key3"}]

With prefix filter:

$ wrangler kv list-keys 06779da6940b431db6e566b4846d64db --prefix taylor
[{"name":"taylor"}]

I made sure that the logic handles pagination correctly for KV namespaces with >1000 keys :)
There are todo comments in my code about particular pieces of logic I'd like to write more elegantly, if possible. I'd love if you could toss ideas there if you have any.

Right now, I print out the results of every page as wrangler gets them (instead of storing them in memory and outputting them all at once). What do the reviewers think about this? I figured this was the best option because it wouldn't eat memory, but I'm curious what other folks think.

This PR will fail CI until cloudflare/cloudflare-rs#30 is merged.