Support hierarchical namespaces in Datafusion integration

[eickler]

Is your feature request related to a problem or challenge?

Currently, while Iceberg and iceberg-rust support hierarchical namespaces, the Datafusion integration in iceberg-rust only makes tables in top-level namespaces available. My use case includes tenancy and fairly complex schemas with thousands of tables, so our lake is quite much nested.

Describe the solution you'd like

A fairly simple addition to the IcebergCatalogProvider can make deeply nested tables using a dot notation (i.e., the namespace is presented as parent.child.… and is available for querying using select … from catalog."parent.child".table). This is apparently also how the same use case is solved in Datafusion itself currently.

Willingness to contribute

I can contribute to this feature independently