iproto: generate iproto_key_type from IPROTO_KEYS

Let's merge the key value type information into IPROTO_KEYS to keep them
close together.

Follow-up #8443
Follow-up commit b3fb883b ("iproto: export IPROTO constants to Lua
automatically")

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring