Edapi

* Add endpoint for user activity
* Add types for user activity responses
* Add `offset` parameter when listing threads to allow pagination
* Modify types to use `Optional` types for backward compatibility
* Add BeautifulSoup4 to dependencies (used in `utils.py` for document parsing)

Initial release for functionality, with the following features:
* Authenticate through an Ed API token (accessible through https://edstem.org/us/settings/api-tokens)
* Create threads
* Edit existing threads (both through global ids and through course-specific ids)
* Uploading files to Ed (through direct file upload)
* Get user information
* List existing threads