Expose/Validate etcd version in the cilium status command

[giorio94]

Historically, Cilium agents validated the version of the etcd endpoints they connected to, expecting v3.1 or subsequent. Yet, this check has been recently dropped, given that the currently supported versions of etcd are only 3.4 (first released in 2019) and 3.5 (first released in 2021), and because it introduced quite a significant overhead at bootstrap time in large setups (both single and multi-cluster environments). Moreover, it is not necessary in combination with the clustermesh-apiserver/kvstoremesh (which is the most common case nowadays), as they run known etcd versions.

This issue is to evaluate the possibility of introducing a one-off check to display and validate the etcd version (as well as possibly other relevant parameters) as part of the cilium status command, or a dedicated sub-command. For instance it could highlight cases in this users are running etcd with known data inconsistency issues (versions 3.5.0-3.5.2).

Suggested by @marseel: cilium/cilium#28834 (comment)