ENH: Add transformation grid sync API/CLI

[snowman2]

• Closes ENH: Replicate projsync for pyproj #572
• Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

Why not use click for the CLI?

• Currently no external dependencies and it is nice that way.
• The CLI isn't all that complex, so not really needed.

Why not use PROJ download methods?

• Only downloads to the user data directory.
• I like adding the .part usage when downloading & checking the file.
• The behavior differed from the grid.available logic in Transformer.download_grids due to it's usage of the cache.db for checking if the download was needed.
• Possibly remove need for requiring PROJ to be compiled with curl if grids pre-downloaded?

[snowman2]

Failures related to this update: OSGeo/PROJ#2237. To be addressed in another PR.