Unitxt 1.15.8

Main changes

Added support for RITS Inference Engine

Inference Engines

• Add inference engines to the catalog by @martinscooper in #1394
• Add support for OpenAI custom base url and default headers + RITS Inference engine by @martinscooper in #1385

Assets

• Add vectara's hhem2.1 faithfulness model as a metric by @lilacheden in #1382

Bug Fixes

• fix template in Arena Hard card and example by @OfirArviv in #1390

Full Changelog: 1.15.7...1.15.8