format: generalize current catalog/schema for arbitrarily nested namespaces

[lidavidm]

What feature or improvement would you like to see?

In theory, if a vendor supports arbitrarily nested table namespaces (a la Iceberg catalog), then we should be able to USE a nested namespace too, right?

• Does any vendor actually support this?
• How should we represent the namespace to the API, considering escaping?

[lidavidm]

I wonder if we need an explicit get/set string list function.

If we want to support this we also need to generalize bulk ingest etc.

[lidavidm]

#46 and #320 already exist; let's use those.