Run `HfApi` methods in the background (`run_as_future`) (#1458)
* POC to run HfApi methods in the background
* Renamed detach to threaded + fix repr
* Complete Refacto
* fix python37
* useless changes
* add test
* update CI
* first step to revert to as_future=True flad
* Make create_commit, upload_file and upload_folder future-compatible
* Finally settle with a solution + tests + docs
* add legacy script for reference
* fix tests
* fix typing in 3.7
* fix typing for 3.7
* minor changes
* make style
* ciode qualiryt