Appender: support nested STRUCTs

[staticlibs]

This PR adds support to nested STRUCT and UNION columns in Appender. There are no nesting limit enforced on Java side.

Usage example:

CREATE TABLE tab1 (
    col1 INTEGER,
    col2 STRUCT (
        s1 INTEGER,
        s2 UNION (
            u1 INTEGER,
            u2 STRUCT (
                us1 INTEGER,
                us2 INTEGER[2],
                us3 VARCHAR
            )
        )
    ),
    col3 VARCHAR
)

try (DuckDBAppender appender = conn.createAppender("tab1")) {
     appender.
        .beginRow()
        .append(45)
        .beginStruct()
        .append(46)
        .beginUnion("u2")
        .beginStruct()
        .append(47)
        .append(new int[] {48, 49})
        .append("bar")
        .endStruct()
        .endUnion()
        .endStruct()
        .append("baz")
        .endRow();
}

Testing: existing test is extended to cover nested STRUCTs.