Unify json library with sttp-ai

[kubinio123]

Currently chimp uses circe while sttp-ai uses upickle. I think we should unify that to either one of them. My personal preference in circe.

[adamw]

Circe pulls in cats, so it's quite heavy. uPickle has problems with customisation. Personally I would prefer to use a single good json library (jsoniter? but the error messages are messy), so maybe circe is the more pragmatic choice.