[RFE] etcd charm should allow configuration of tuning options

[Patrizio Bassi]

In https://charmhub.io/etcd/configure i can't find any reference to tuning parameters as documented in the upstream https://etcd.io/docs/v3.5/tuning/ page

in particular
heartbeat-interval (default 100 ms)
election-timeout (default 1000 ms)
snapshot-count (default 10000)

values are in ms

in some cases, for instance in our infrastructure with ceph distributed backend, these values may result a bit too tight which will lead to frequent leader re-election and K8s APIs outage

Thank you