**Describe the solution you'd like**

We already can run clickhouse-server without configured ZooKeeper (the tables will start in read-only mode)
and with configured ZooKeeper, if it starts to be inaccessible at runtime (the tables will be read-only until the session is re-established).

Let's also make it possible to start up when ZooKeeper is unavailable.

**Additional context**

If that will be very hard to implement, we can consider other options:
- lazy attaching of the tables after sever start-up;
- a maintenance mode when the tables are not attached at start-up and require manual ATTACH queries.
const IMergeTreeDataPart & part,
    const DiskPtr & disk,
    const String & path)
{
    const auto settings = getSettings();
    auto data_source_description = disk->getDataSourceDescription();
    if (!(disk->supportZeroCopyReplication() && settings->allow_remote_fs_zero_copy_replication))
        return nullptr;
    String replica = getSharedDataReplica(part, data_source_description.type);
    
    if (replica.empty())
        return nullptr;
    return executeFetchShared(replica, part.name, disk, path);
}
String StorageReplicatedMergeTree::getSharedDataReplica(