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ARROW-15267: [GLib] Add garrow_function_get_default_options()

The following functions are also added for testing:

  * garrow_function_options_equal()
  * garrow_function_options_to_string()

Closes #12087 from kou/glib-function-default-options

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