mkversion: introduce get_latest method

mkversion.get_latest() returns the latest schema version known to this
tarantool build. It is going to be used in config module to determine
when dd operations are allowed.

It will be used in places where `schema_needs_upgrade()` can't be used,
because it compares only the current schema version, not any version you
pass to it.

In-scope-of #9502

NO_DOC=internal
NO_TEST=covered by the next commit
NO_CHANGELOG=internal