Reapply() is supposed to make sure that the system (the interface)
is configured as indicated by the applied-connection. That means,
it will remove/add configuration to make the system match the requested
configuration.
Add a flag "preserve-external-ip" which relaxes this. During reapply,
IP addresses/routes that exist on the interface and which are not known
(or added) by NetworkManager will be left alone.
This will be used by nm-cloud-setup, so that it can reconfigure the
interface in a less destructive way, which does not conflict with
external `ip addr/route` calls.
Note that the previous commit just adds "VersionInfo" and the
possibility to expose capabilities (patch-level). This is not used
for the new reapply flag, because, while we might backport the
reapply flag, we won't backport the "VersionInfo" property. Exposing
new capabilities via the "VersionInfo" property will only become useful
in the future, where we can backport a capability to older NM versions
(but those that have "VersionInfo" too).
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