Add cluster join API version check

Add cluster join API version check

Adds API call GET /cluster/config/apiversion to retrieve remote clusters
join-API version (0 is assumed for versions without this endpoint). Also
available via CLI as 'pvecm apiver'.

Introduce API_AGE similar to storage plugin API, but with two ages for
cluster/joinee roles. Currently, all versions are intercompatible.

For future usage, a new 'addnode' parameter 'apiversion' is introduced,
to allow introducing API breakages for joining nodes as well.

As a first compatibility check, use new fallback method only if
available. This ensures full compatibility between nodes/clusters with
and without new fallback behaviour.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>