ci: assigner: add a function to assign issues

Add a -I option to process an issue and, if it's not assigned already,
add assignees based on the labels, matching the ones set in the
MAINTAINERS file areas.

Suggested-by: Henrik Brix Andersen <henrik@brixandersen.dk>
Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>