plugin upload, XEP-0363: code modernisation + preparation for extension: - use of async/await syntax - fileUpload's options are now serialised, allowing non string values - (XEP-0363) Slot is now a dataclass, so it can be modified by other plugins - (XEP-0363) Moved SSL related code to the new tools.web module - (XEP-0363) added `XEP-0363_upload_size` and `XEP-0363_upload` trigger points - a Deferred is not used anymore for `progress_id`, the value is directly returned