tarantool-etcd-7ey Replica route: connect as read-only replica to an existing etcd cluster Lined Up

Add a mode where this server attaches to an existing/real etcd cluster as a read-only replica (mirror), serving reads locally while transparently keeping in sync with the upstream etcd.

Motivation: lets the Tarantool store act as a read scale-out / local cache in front of a real etcd deployment, and is a stepping stone toward migration (shadow the production cluster, compare, then cut over).

Design questions to resolve:
- Sync mechanism: use etcd's Watch (clientv3) from revision 0 to stream the full keyspace + tail live, persisting events into our etcd_* spaces and advancing our revision counter to match upstream. Need an initial Range snapshot (or Snapshot RPC) for bootstrap, then Watch with progress notifications + compaction handling (ErrCompacted -> re-snapshot).
- Revision alignment: our global revision sequence must track upstream's mod_revision exactly so reads return etcd-consistent revisions. This constrains the local revision allocator while in replica mode.
- Write handling: reject writes locally with a redirect/error, OR proxy them to the upstream leader and wait for the change to come back over the watch (read-your-writes). Pick one; proxying is closer to etcd's learner/proxy semantics.
- Lease/Watch/Auth: leases and auth state also need mirroring or pass-through. Scope what's in v1 (KV-only read replica) vs later.
- Config: new role config block (e.g. replica.upstream_endpoints, replica.mode = mirror|proxy, credentials).

Likely a multi-phase epic; this ticket captures the umbrella. Split bootstrap-snapshot, watch-tail-sync, revision-alignment, and write-proxying into child issues once the design is settled.

Reuses clientv3 Watch semantics we already understand from conformance work (ErrCompacted=11, progress notify). Consider relation to the Tarantool-native replicaset path (tarantool_rs3) — this is the opposite direction (follow a foreign etcd, not our own Raft).