Add http4s 0.23.x Generator

[barryoneill]

Http4s 0.23.x is based on Cats Effect 3.x and as such, all transitive libraries are now CE3 based as well.

The current 0.22.x client generated will not compile for http4s 0.23. The CE3 typeclass hierarchy has changed, and the default Sync bound that is currently generated makes Concurrent unavailable to circe (which it requires).

I've only looked at the client so far, but it looks like changing the selected bound from Sync to Concurrent in a new 0.23 generator will resolve the issue. Will look at the server next.

[barryoneill]

Created https://github.com/barryoneill/apibuilder-http4s-test to show the issue

Also noticed that perhaps the Server implementations are not working - but will test that out later. @kovszilard has offered to help with this one.