Support Entra ID authentication for Direct Line

[adilei]

Summary

Add support for authenticated agents that require Entra ID (Azure AD) tokens to establish a Direct Line conversation.

Motivation

Many enterprise bots require user authentication via Entra ID before they can interact. Currently the library only supports Copilot Studio agents configured with "No authentication". Supporting Entra ID would unlock enterprise scenarios where:

• The agent needs to know the user's identity
• The agent calls downstream APIs on behalf of the user (OBO flow)
• The organization requires authenticated access to the bot

Possible Approach

• Add a data-webchat-auth attribute (e.g., entra)
• Add data-webchat-client-id and data-webchat-tenant-id attributes
• Use MSAL.js to acquire a token, then exchange it for a Direct Line token
• Handle token refresh for long-running conversations