Apache Iceberg Rust version

None

Describe the bug

Other implementations of list_namespace(parent) return absolute namespace identifiers. For example, if there is a namespace b under a, list_namespace("a") should return NamespaceIdent::from_strs(vec!["a", "b"]). This is documented in the REST OpenAPI spec, the Java implementation and, in this repo, in SqlCatalog.

MemoryCatalog returns relative identifiers, as can be seen here. To make MemoryCatalog mimick real implementations better, the behaviour should be fixed.

To Reproduce

Current behaviour is demonstrated in a MemoryCatalog unit test.

Expected behavior

Desired behaviour is demonstrated in the matching SqlCatalog unit test.

Willingness to contribute

I can contribute a fix for this bug independently