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ARROW-15205: [GLib] Add garrow_function_all()

Some convenience functions for testing are also added:

  * garrow_function_get_name()
  * garrow_function_equal()
  * garrow_function_to_string()

Closes #12056 from kou/glib-function-all

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