Expose COPY method

[yourzbuddha]

Hey, I have a need for the copy method, which is currently not exposed.
I want to contribute to it, but first, I want to understand if it aligns with the lib's vision.

@profunctor WDYT?

[gvolpe]

I see nothing wrong with adding that method, can be done here: https://github.com/profunktor/redis4cats/blob/series/1.x/modules/effects/src/main/scala/dev/profunktor/redis4cats/algebra/keys.scala#L26

In the meantime, users can use the unsafe API for any unsupported command that is supported by Lettuce.