[Variant] Panic when appending Object or List to VariantBuilder

[alamb]

Describe the bug
The VariantBuilder code will panic if you try and append an existing Object or LIst

To Reproduce
Add this test to parquet-variant/src/builder.rs"

    #[test]
    fn test_append_object() {
        let (object_metadata, object_value) = {
            let mut builder = VariantBuilder::new();
            let mut obj = builder.new_object();
            obj.append_value("name", "John");
            obj.finish();
            builder.finish()
        };
        let object_variant = Variant::try_new(&object_metadata, &object_value).unwrap();

        let mut builder = VariantBuilder::new();
        builder.append_value(object_variant.clone());
        let (metadata, value) = builder.finish();
        let variant = Variant::try_new(&metadata, &value).unwrap();
        assert_eq!(variant, object_variant);
    }
}

This results in

internal error: entered unreachable code: Object and List variants cannot be created through Into<Variant>
thread 'builder::tests::test_append_object' panicked at parquet-variant/src/builder.rs:395:17:

Expected behavior
I expect the object variant to be copied to the in progress builder

Additional context

[Weijun-H]

take

[alamb]

label_issue.py automatically added labels {'parquet'} from #7735