feat: add {{USER_GROUPS}} and {{USER_GROUP_IDS}} placeholders for custom forwarded headers (#27236)

Custom per-connection headers can now forward the user's groups to
upstream backends via two new template placeholders:

- {{USER_GROUPS}}: comma-separated group names
- {{USER_GROUP_IDS}}: comma-separated group ids

The group lookup is async, so get_custom_headers becomes an async
wrapper around the sync template substitution (parse_custom_headers)
and fetches groups lazily — only when a header value actually
references a groups placeholder. The external document loader path
runs in a worker thread without an event loop, so Loader.aload
prefetches the groups before offloading and passes them through to
ExternalDocumentLoader.


Claude-Session: https://claude.ai/code/session_01EbBEfTyu8fFJmC13rnQthT

Co-authored-by: Claude <noreply@anthropic.com>

backend/open_webui/utils/tools.py

@@ -168,7 +168,7 @@ async def build_tool_server_headers(
     # Interpolate template vars in custom connection headers
     connection_headers = connection.get('headers', None)
     if connection_headers and isinstance(connection_headers, dict):
-        headers.update(get_custom_headers(connection_headers, user, metadata))
+        headers.update(await get_custom_headers(connection_headers, user, metadata))
 
     # Add user info headers if enabled
     if ENABLE_FORWARD_USER_INFO_HEADERS and user: