The Webfinger endpoint is always under /.well-known/webfinger, and it receives queries such as /.well-known/webfinger?resource=acct:bob@my-example.com. Well, in our case we can cheat, and just make it a static file:

This specification defines the WebFinger protocol, which can be used to discover information about people or other entities on the Internet using standard HTTP methods. WebFinger discovers information for a URI that might not be usable as a locator otherwise, such as account or email URIs.