Add endpoint to read values from KV

[caass]

The motivation here is to backport cloudflare/workers-sdk#587, so we need to be able to read KV values so that we can re-upload them with a TTL.

There is some issue here in that the expiration on a KV pair is sent back as a response header, which as far as I can tell we don't have a way to incorporate into this library. Would love it though if it were possible to deserialize the TTL from the response header!

Draft for now since I still need to test if this actually works, but serde_json supports deserializing raw strings:

fn main() {
    let hello: String = serde_json::from_str("\"hello\"").unwrap();
    println!("{hello}")
}

   Compiling json-string v0.1.0 (/Users/cass/Projects/work/json-string)
    Finished dev [unoptimized + debuginfo] target(s) in 1.22s
     Running `target/debug/json-string`
hello

The API Docs indicate that KV just returns a raw string, so...it should work? But I'm going to test it out before marking this PR as ready for review.

[Noah-Kennedy]

@caass you also need to fix some clippy lints

[caass]

Ah it was too good to be true anyway, I can't seem to make it work.

[Noah-Kennedy]

Ah, sad