Glib has GValue which used for boxing value.

Add NMValueType enum, which has a similar purpose, but it's much more
limited.

- contrary to GValue, the type must be tracked separately from the
  user-data. That is, the "user-data" is only a pointer of appropriate
  type, and the knowledge of the actual NMValueType is kept separately.
  This will be used to have a static list of meta-data that knows the
  value types, but keeping the values independent of this type
  information. With GValue this would not be possible.

- the use case is much more limited. Just support basic integers,
  boolean and strings. Nothing fancy.

Note that we already do something similar at muliple places. See for
example NMVariantAttributeSpec and nm_utils_team_link_watcher_to_string().
These could/should instead use NMValueType.