When comparing the Python implementation of SqlCatalog with the Java implementation (JdbcCatalog), I have noticed some differences that this PR tries to address. In my opinion, we should be able to read from the same catalog database, using both the Java or Python implementations with no impact.

Here is a list of changes proposed by this PR:

• The Java implementation supports hierarchical namespaces, or multi-level namespaces separated by .. For example, ns1.ns2.ns3 is a valid namespace in Java, but not with the current Python implementation. Identifiers will now have an optional catalog_name, zero or more namespaces and a table_name. For example, an identifier for table catalog_name.ns1.ns2.ns3.table_name will be represented as the following tuple: ("catalog_name", "ns1", "ns2", "ns3", "table_name")
• create_table(), register_table() now supports a table identifier with an optional catalog name.
• References to database_name have been generalized by namespaces.
• Fix a bug in list CLI command
• Adds new and extends many unit tests to run against hierarchical namespaces.

Here is a list of things to do before opening PR:

• Validate that the list CLI command fix is good
• Should we allow table identifiers to have no namespace at all, like we do in Java? (pending @Fokko ) No.