Refactor `ItfCounterexampleWriter`

[konnov]

We should refactor ItfCounterexampleWriter.scala:

• move the logic from the object to the package at.forsyte.apalache.io.itf and introduce class TlaToItfJson, similar to ItfJsonToTla.
• parameterize this class by JsonRepresentation, so we can use different JSON serializers.

This is needed to avoid the roundtrip of UJson <-> Jackson, which would also allow us to get rid of UJson in the future, if needed. We can do that once #3212 is in.