Refactor ChatSkill to use new Chat Completions prompt format

[teresaqhoang]

Motivation and Context

This PR refactors the ChatSkill to use the new ChatCompletions prompt format as the meta-prompt template. The new format serves the prompt template as a series of context messages, where each message can be tagged a role, to prime the model in a certain way. These roles include: System, User, and Assistant.

Description

For ChatSkill, the prompt template for the meta-bot response request utilizes the message roles in the following way:

• System: for any instructions or additional context provided to the model, including bot persona, relevant memories, or planner output.
• Assistant: Any bot response message in chat history previously outputted as chat completion result.
• User: Any user input.

The series of messages between the user and the assistant is included both as few shot examples and additional context (chat history). The last user message is always included.

Other changes:

• This PR also adds a custom token calculator, since chat streaming doesn't return token usage details.
• System continuation preamble is no longer needed with this new format, since it's been optimized for chat responses.
• Added visible scroll bar to prompt dialog.
• Simplified SystemResponse since this conversational interface is already optimized for chat.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines and the pre-submission formatting script raises no violations
  - [ ] All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄