create-spdx: add create_annotation function

This allows code reuse and future usage with relationship annotations

Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 1f8fdb7d)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>