Support multiple namespaces within catalogs

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

Current way to identify tables/relations: `catalog.schema.table`

Be able to support more than 1 schema (aka namespace), e.g. `catalog.namespace1.namespace2.namespace3.table`

**Describe the solution you'd like**

Be able to support querying catalogs that support nested namespaces, e.g. for possible future Apache Iceberg integration

Other engines like Apache Spark also supports this

**Describe alternatives you've considered**

Don't do it (I don't believe Postgres supports it)

**Additional context**

Iceberg reference: https://iceberg.apache.org/docs/latest/spark-queries/#catalogs-with-dataframereader

Start by reviewing DataFusion's catalog, schema, and table naming model, then compare the requested nested namespace form with the linked Iceberg reference and Spark behavior. Define how catalog.namespace1.namespace2.namespace3.table should be represented and resolved while preserving existing three-part names. Add coverage for nested namespaces and verify querying works as requested.

Written by the indexing model from the issue text.