feat/enh: api keys user permission

breaking change, `ENABLE_API_KEY` renamed to `ENABLE_API_KEYS` and disabled by default and must be explicitly toggled on.

backend/open_webui/routers/users.py

@@ -208,6 +208,7 @@ class ChatPermissions(BaseModel):
 
 
 class FeaturesPermissions(BaseModel):
+    api_keys: bool = False
     direct_tool_servers: bool = False
     web_search: bool = True
     image_generation: bool = True