Add well-known route for Device Bound Session Credentials (#54328)

This supports the Web Platform Tests for session registration from a
subdomain being added in https://crrev.com/c/6847053.

Co-authored-by: Daniel Rubery <drubery@chromium.org>

Author: drubery

tools/serve/serve.py

@@ -801,6 +801,7 @@ def add_mount_point(self, url_base, path):
             ("*", "/.well-known/private-aggregation/*", handlers.PythonScriptHandler),
             ("GET", "/.well-known/shared-storage/trusted-origins", handlers.PythonScriptHandler),
             ("*", "/.well-known/web-identity", handlers.PythonScriptHandler),
+            ("*", "/.well-known/device-bound-sessions", handlers.PythonScriptHandler),
             ("*", "*.py", handlers.PythonScriptHandler),
             ("GET", "*", handlers.FileHandler)
         ]