Add result upload and download feature

[anhappdev]

• The app can upload result files to a cloud storage (feat: add result upload #703)
• The app can download result files from a cloud storage (only results from the same user_id) (feat: add result download #752)
• Provide option to convert from an anonymous account to a normal account (feat: add user signup flow #757)

Background:
In the past, we tried to use the Firebase Database for storing results but it has very limited capabilities in filtering (see #490). Since we don't expect the data (the result) to be modified, and the amount of data will not be big, we will be fine with just storing the result as a json/text file, then upload it to a cloud storage, and download other results from the cloud storage and do all the filtering locally on-device. The filtering is already implemented in #641.