feat: Enable `phoenix.evals` to handle multimodal message templates (#5522)

* Templates return lists of messages from `format` method

* Update model wrapper base class

* Migrate model wrappers

* Resolve type errors

* Fix more type issues

* Wire up full backwards compatibility

* Do not error not on PromptMessage inputs to model wrappers

* Do not use list subscripting

* Refactor to use generic `parts` instead of `messages`

* Ruff 🐶