Add bot message instruction

[bongole]

What is this PR:

• [ ] Bug fix
• [x] Feature
• [ ] Chore

Description: In this PR, it is possible to insert instruction at the stage where the Bot generates messages. The usage of this feature intend to scenarios such as forcing a language of the Bot's message.

For example:

import logging
from nemoguardrails import LLMRails, RailsConfig

logging.basicConfig(level=logging.DEBUG)

COLANG_CONFIG = """
define user express greeting
  "hello"

define flow greeting
  user express greeting
  bot express greeting
  bot ask how are you
"""

YAML_CONFIG = """
instructions:
  - type: bot_message
    content: |
      BOT MUST RESPOND ONLY IN JAPANESE AND JAPANESE CHARACTERS.

models:
  - type: main
    engine: openai
    model: gpt-3.5-turbo
"""

def demo():
    config = RailsConfig.from_content(COLANG_CONFIG, YAML_CONFIG)
    app = LLMRails(config=config, verbose=True)

    history = []

    while True:
        user_message = input("> ")

        history.append({"role": "user", "content": user_message})
        bot_message = app.generate(messages=history)
        history.append(bot_message)
        print(history)

        print(f"\033[92m{bot_message['content']}\033[0m")


if __name__ == "__main__":
    demo()

Example reuslt:

  user> hello
  bot> こんにちは！今日は何かお手伝いできますか？ お元気ですか？