[feature request] create new users for easy bridging with Matterbridge

It would be nice if this could create new users on the fly based on user name and avatar url provided in a webhook.

This way it would be relatively easy to simulate real matrix users with Matterbridge, similar to how it works with Slack etc.

Adding appservice support directly to Matterbridge is sadly unrealistic right now, so this would be the next best option.

Thanks.

The issue names no files or tests. Start by locating webhook handling and the existing user or identity-management entry points, then trace how incoming usernames and avatar URLs are represented. Done means the bridge can create users on demand from those webhook values and supports the Matterbridge-style flow described, with coverage for the new behavior.

Written by the indexing model from the issue text.