Support default storage location

[Jakuje]

          Should we add a default place to look at when no env var is set ?

(perhaps in a separate PR)

Originally posted by @simo5 in https://github.com/latchset/kryoptic/issues/35#issuecomment-2066696699

This was discussed previously and we considered some defaults like:

• if root: some system wide location
• else: try XDG_CONFIG_HOME [1] or something like that

The environment variable and pReserved argument to Initialize should override the defaults.

[1] https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html