Add multi api inference engine

[elronbandel]

With multi api inference engine you can feed into your application:

     MyClass(
           model=CrossProviderInferenceEngine("llama-3.1-405b", api="aws")
      )

Then change it by editing the model.api argument or by general settings: You can set unitxt.settings.default_provider="watsonx" or export UNITXT_DEFAULT_PROVIDER="watsonx"