arrow

ARROW-15203: [GLib] garrow_struct_scalar_get_value() for scalar from C++ returns value

We need to convert C++ scalars lazy because there is no change to
convert C++ scalars to GLib scalars.

Closes #12041 from kou/glib-struct-scalar-get-value

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>