Update the `openai-gpt` agent to get it ready for first-time users

[daxian-dbw]

PR Summary

Fix #128, fix #129, fix #145

• Removed the https://pscopilot.azure-api.net related code and example setting file. Redesigned the UX for first-time user.
• Updated the openai-gpt agent to better detect setting file changes and auto-update the settings in that case.
• Fixed /gpt and /agent commands to prevent null-ref exception when no GPT is defined, or no agent is defined.
• Update the README.md for openai-gpt.