Deno.Kv: ability to retrieve just keys

[kitsonk]

Related to #18965

There feels like there is a use case for just enumerating the keys of a .list() function, like for example to be able to retrieve and then delete a set of keys, without needing to allocate all of the values in the runtimes memory.

[miguelrk]

Any news on this? Not only is having to use .list() suboptimal performance wise (having to read keys AND values into memory) but also cost wise (on Deploy). +1 to have an operator for this built-in.