ENH: Replicate projsync for pyproj

[snowman2]

Related to: OSGeo/PROJ#2028

Two parts:

1. Python API
2. CLI

Currently, without pj_context_set_user_writable_directory, this means that the download can only go to the user writable directory via the PROJ API.

The alternative would be to create a python based downloader, but the logic may differ from PROJ.

• To get the user writable directory, could use https://github.com/ActiveState/appdirs
• To download the file, have many options available and can manually download to user directory, the data directory, or whichever is specified by the user.