Support custom token endpoints (non-Copilot Studio)

Summary

Add support for custom token endpoints that are not Copilot Studio, such as a server-side proxy that holds a Direct Line secret and returns conversation tokens.

Motivation

Currently the library assumes the token endpoint is a Copilot Studio endpoint and auto-detects the regional Direct Line domain via regionalchannelsettings. Users with custom token endpoints (e.g., their own backend that holds a Direct Line secret and generates tokens) can already use data-webchat-token-url, but the regional domain auto-detection won't work since there's no regionalchannelsettings API.

Proposed Change

Add a data-webchat-domain attribute that lets users explicitly set the Direct Line endpoint URL (e.g., https://europe.directline.botframework.com/v3/directline). When set, this overrides the auto-detected regional domain.

This is the only missing piece — the token endpoint pattern already works. Users should keep their Direct Line secret on the server and expose a token endpoint, not put secrets in HTML.

Note on Direct Line secrets

This library will not support putting Direct Line secrets in HTML attributes. Secrets are long-lived server-side credentials meant for secure environments (key vaults, backend services). The correct pattern for client-side usage is always a token endpoint that returns short-lived conversation tokens.

Start by locating the embed SDK handling for data-webchat-token-url and regionalchannelsettings, then trace how the Direct Line domain is selected. Add the proposed data-webchat-domain configuration and verify that it overrides regional auto-detection while the existing custom token endpoint flow continues to work.