Naming

In order to migrate out of decommissioned in grpc-1.30 APIs of naming (and picker),
still addressing the use-cases documented in:
https://github.com/etcd-io/etcd/blob/master/Documentation/dev-guide/grpc_naming.md

We execute following plan:

1. Introduce new interfaces as defined in: Replacement API for client/v3/naming package to be compatible with new GRPC1.30+ resolver API. #12614
2. Implement them [help needed]
3. Migrate grpc proxy on the new implementation. This is hopefully ready:(ptabor@142b9a3)

Picker

Apart of dealing with clientv3/naming as documented above,
we need to migrate clientv3/balancer on a new picker.
The code from contributions like:
#12384
#12636
#12439
Might be good for this (when isolated and passing test).
Mid term I would think why/whether etcd needs own balancer and why just:
https://pkg.go.dev/google.golang.org/grpc@v1.35.0/balancer/roundrobin
on top of some 'passthrough' like https://pkg.go.dev/google.golang.org/grpc/resolver