plugin loader: add an APIAGE

plugin loader: add an APIAGE

With the addition of the map/unmap_volume() methods we made
an (actually unnecessary) API version bump.
All current users of these methods fall back to path() when
they return undef, so plugins implementing version 1 are
in fact compatible currently. (In fact, the default
Plugin::map_volume() could fall back to it on its own, but
doesn't currently).

For now let's just allow plugins older plugins to also be
loaded by introducing an API age variable. With it, if we
have a reason to break older plugins, we can have a
deprecation period during which older plugins cause a
warning instead of refusing to load altogether.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>