Downloading grids during runtime & updating available grids

[snowman2]

From: pyproj4/pyproj#1474

Each context has a cache of available grids it generates once and doesn't look for grids after that for performance. However, some programs download grids during runtime. Having a way to update the cache or clear it would help with this use case.

Ideas:

• Add proj_reset_found_grids to invalidate cache or for a specific grid.
• Haveproj_download_file invalidate the cache or for a specific grid.

[snowman2]

The first option would be helpful for programs that have an alternative method to download the grid files. pyproj currently uses a python function to download the file.