connection: Support ip-ping-target and ip-ping-timeout

Summary

connection: Support ip-ping-target and ip-ping-timeout

Purpose

We have encountered multiple incidents where users face connectivity
issues after booting, particularly due to hardware like switches that do
not pass traffic for a few seconds after startup. And services such as
NFS fail to mount because they try to initiate before the network is
fully reachable. Current settings like gateway-ping-timeout do not
always resolve the issue, as the gateway may not always be part of the
connection path in these cases. Therefore, we are supporting
`connection.ip-ping-target` and `connection.ip-ping-timeout` to allow
administrators to configure the network to verify connectivity to a
specific target (such as a service like NFS) instead of relying on
gateway reachability, which may not always be relevant in certain
network configurations.

Resolves: https://issues.redhat.com/browse/RHEL-21160

Checklist

Please read https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/CONTRIBUTING.md before opening the merge request. In particular, check that:

• the subject for all commits is concise and explicative
• the message for all commits explains the reason for the change
• the source is properly formatted
• any relevant documentation is up to date
• you have added unit tests if applicable
• the NEWS file is updated when the change deserves to be mentioned, for example for new features, behavior changes, API deprecations, etc.