New PromptBuilder returning list of messages

[javipacheco]

This PR changes the way to make prompts. This is the new way:

      val messages = buildPrompt {
        +system("Test System")
        +user("Test Query")
        +assistant("Test Assistant")
      }

buildPrompt returns a List<Message> that you can pass to prompt functions in the conversation scope.

This PR removes:

• ExpertSystem
• Old Message model that we are not using in xef.prompt package
• The suspend functions in the Message companion object

[raulraja]

Looks great @javipacheco