Refactored the authentication flow and add the check for Copilot access

[daxian-dbw]

PR Summary

Refactored the authentication flow and add the check for Copilot access.

• Encapsulated the token life cycle management into separate classes UserAccessToken and UserDirectLineToken. The health of the token is tracked within those types, which also expose the renew methods.
  • This is a preparation for using the pluginstore feature flag, which requires the client to send user access token along with every request, and that means we need to keep 2 tokens alive.
• Refactored the authentication flow using the new encapsulated tokens.
• Add the step to check if the user has Copilot access.