[REQ][Finch] Add Cats option to Finch server codegen

Created by: lbordowitz

Is your feature request related to a problem? Please describe.

In order to use the Scala library Cats, endpoint code generation for the Finch library is different. For example, while ordinary Finch usage defines an endpoint of the type Endpoint[io.circe.Json], we can also explicitly invoke the IO monad with Endpoint[cats.effect.IO, io.circe.Json] (full package names added for context, they would normally just be imported). The code within the endpoint would also look different.

Describe the solution you'd like

We should provide an option to allow an explicit cats.effect usage of Finch, for those users who have services and other downstream dependencies wrapped with cats-style code.

Describe alternatives you've considered

I've considered making a private library, or creating a new public generator. An option with the existing Finch code generator would be preferable.

Additional context

For example openapi-codegen generate -i [file] -l finch --cats -o /tmp/test/

I'm not sure whether this is available for v2.