One external code a GeoGenius.Provider.Area carries, such as a FIPS or
GNIS identifier.
code_type is a plain string, not a closed atom like Name.kind -- code
types are open-ended (FIPS, GNIS, ZIP, a source's own postal code, and
whatever the next authority calls its own identifier), so there is no
fixed set to close over. That means a provider that misspells a code type
gets no Dialyzer protection the way a misspelled Name.kind atom would;
a typo here surfaces only as a code nothing queries for, at runtime.