feat: LiteLLM model support for evals (#1675)

* LiteLLM implmentation for evals

* LiteLLM completion retry

Support for LiteLLM
Encoding interface return type
Tokenizers added as dep

* type checking

Removed encoder implementation, unnecessary.

* Done

* better init validation, mock pytest, docs

Added litellm to dev & hatch envs dependencies

* dear mypy

* have to ignore

"litellm": module is installed, but missing library stubs or py.typed marker

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Co-authored-by: Mikyo King <mikyo@arize.com>