This function downloads that database, verifies its SHA256 checksum,
and, on R 4.0 or newer, caches it under
tools::R_user_dir("zipcodeR", "data"); later calls return the cached
path immediately. On older supported R versions the verified file is kept
only in the session temporary directory. It never downloads without being
called explicitly. For offline use on R 4.0 or newer, copy the file to that
user data directory yourself (the expected file name is the asset name from
the data release).