Flatten messages in PromptBuilder

[javipacheco]

This PR closes issue #423 for flattening messages in the PromBuilder

When we have a prompt like this

        Prompt {
            +system("Test System")
            +user("User message 1")
            +user("User message 2")
            +assistant("Assistant message 1")
            +user("User message 3")
            +assistant("Assistant message 2")
            +assistant("Assistant message 3")
            +user("User message 4")
          }

Prompt flattens the intermediate messages with the same role automatically in order to reduce the number of tokens to the AI

[raulraja]

This should be the default behavior and not an operator

[javipacheco]

@raulraja I have addressed your proposal in the PR. Could you please review again?

[Intex32]

Me approves.

[raulraja]

thanks @javipacheco !