Update TaskCompletion Evaluator to support multi-turn evaluation (#4922)

* Update TaskCompletion to support multi-turn eval

* run code health

* Update multi-turn validation, change session term to conversation, add tests and comments

* Add optional evaluation_mode parameter

* update tests

* code health

* update conversation parameter to messages

* add supported_evaluation_level

* pop messages in single-turn path

* add messages to the optional params

* Add preview tag, update test

* Rename supported evaluation level to evaluation level

* add reformatting tests

* remove preview tag, update spec

* update tests

* Update supported_evaluation_level, use constants for roles

Author: salma-elshafey