### What problem does this PR solve?

Iceberg has 3 levels of metadata: catalog, namespace and table, mapping
to Doris' catalog, database and table.

Iceberg support nested namespaces, which means the following namespaces
are valid:
```
ns1
ns1.ns2
ns1.ns2.ns3
```

So we need to support mapping nested namespace to Doris' database.

This PR add a global variable `enable_nested_namespace` to control this
behavior.
Default is `false`, and no logic is changed.

If set to true, Doris can support following statments:

```
mysql> switch iceberg;
mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| nested             |
| nested.db1         |
| nested.db2         |
+--------------------+

mysql> use iceberg.nested.db1;
ERROR 1049 (42000): Only one dot can be in the name: iceberg.nested.db1
mysql> use iceberg.`nested.db1`;
ERROR 5086 (42000): errCode = 2, detailMessage = Unknown catalog 'nested'

mysql> set global enable_nested_namespace=true;

mysql> use iceberg.nested.db1;
Database changed
mysql> select k1 from iceberg.`nested.db1`.nested1;
mysql> select nested1.k1 from `nested.db1`.nested1;
mysql> select `nested.db1`.nested1.k1 from iceberg.`nested.db1`.nested1;
mysql> select iceberg.`nested.db1`.nested1.k1 from nested1;
+------+
| k1   |
+------+
|    1 |
+------+

mysql> refresh catalog iceberg;
mysql> refresh database iceberg.`nested.db1`;
mysql> refresh table iceberg.`nested.db1`.nested1;
Query OK, 0 rows affected (0.01 sec)
```

But, I can execute statement like:
```
use iceberg.`nested.db1`;
```

I don't know why, there is a very strange behavior in MySQL client, when
adding back quota,
the INIT_DB command can only receive `nested.db1` part, but expect
`iceberg.nested.db1`.

Also support creating nested database name in internal catalog:
```
create database `db1.db2`
```