**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 ASTPtr & partition,
    const StorageMetadataPtr & metadata_snapshot,
    bool attach_part,
    ContextPtr query_context)
{
    
    if (!are_restoring_replica)
        assertNotReadonly();
    PartitionCommandsResultInfo results;
    PartsTemporaryRename renamed_parts(*this, "detached/");
    MutableDataPartsVector loaded_parts = tryLoadPartsToAttach(partition, attach_part, query_context, renamed_parts);
    
    ReplicatedMergeTreeSink output(*this, metadata_snapshot, 0, 0, 0, false, false, query_context,
        true);
    for (size_t i = 0; i < loaded_parts.size(); ++i)
    {
        const String old_name = loaded_parts[i]->name;
        output.writeExistingPart(loaded_parts[i]);
        renamed_parts.old_and_new_names[i].old_name.clear();
        LOG_DEBUG(log, "Attached part {} as {}", old_name, loaded_parts[i]->name);
        results.push_back(PartitionCommandResultInfo{
            .partition_id = loaded_parts[i]->info.partition_id,
            .part_name = loaded_parts[i]->name,
            .old_part_name = old_name,
        });
    }
    return results;
}
void StorageReplicatedMergeTree::checkTableCanBeDropped() const