Add `KeyDecoder`/`KeyEncoder` to Circe integrations

[satorg]

Introduces DerivedCirceKeyCodec along with DerivedCirceKeyDecoder and DerivedCirceKeyEncoder to provide support for keys in Circe.

Seems to be a pretty straightforward addition. The code mostly follows the pattern in DerivedCirceCodec and its counterparts.

[satorg]

I've just realized that I missed this suggestion from "CONTRIBUTING.md"

2. Before starting to work, make sure there is a ticket in the issue or create one first. It can help accelerate the acceptance process if the change is agreed upon

I can file it if it is still required. Sorry for my negligence.

Also it doesn't seem there's any "AUTHORS" file to meet this one:

5. Your first commit request should be accompanied with a change to the AUTHORS file, adding yourself to the authors list.

[alexandru]

Thanks for the PR, and sorry for the latency, I'm looking at it.