Interval fields returned as `String`

### Description

DuckDB supports interval types, but the DuckDB JDBC Driver returns these fields as `String`. In comparison the PostgreSQL JDBC Driver returns a custom [`org.postgresql.util.PGInterval`](https://jdbc.postgresql.org/documentation/publicapi/org/postgresql/util/PGInterval.html) instance.

### Expectation

This makes the integrations for the DuckDB JDBC Driver problematic in certain situations. I would expect a custom class to be returned similar to the `org.duckdb.user.DuckDBUserArray`, `org.duckdb.user.DuckDBMap`, or `org.duckdb.user.DuckDBUserStruct` classes.

Even if this new class behaves the same as a `String` instance, using a different class would make it possible for the implementations to convert that to a proper `Duration` type at the end, or whatever the end-user prefers.

### Repro case

https://github.com/raszi/duckdb-interval-testcase