Nil pointer dereference in ETCD config parsing for TT 3.x

[hackallcode]

2.0.1, tt 3.0, viper config

[KaymeKaydex]

looks like https://github.com/tarantool/go-vshard-router/blob/master/providers/viper/tarantool3/convert.go#L10C20-L10C25 Replicasets is nil pointer
anyway we need to check this

[KaymeKaydex]

Panics were initially built into the provider for incorrect parsing, it seems this may not lead to unexpected consequences. But readability will be improved in #37

[palage4a]

I encountered a similar issue. I believe the problem is that the group names are hardcoded as routers and storages, whereas the cluster configuration allows for any arbitrary names.

Additionally, based on the code, it seems mandatory to split routers and storages into separate groups, as this is how the go-vshard-router determines the role of a replicaset.

PS tarantool/tt has a lib which provides some cluster configuration possibilities. Might be useful https://github.com/tarantool/tt/blob/f13b13e8f7813e1e95765e8c3e93fdd27380bbca/lib/cluster/cluster.go#L123C1-L167C2