CI: implement scheduled auto-update pipeline (MR 4967)

Introduce support for having CI automatically update packages using
project access tokens and scheduled pipelines.

The pipeline schedule is configured with an additional variable
containing a list of packages to update, the autoupdate-fetch.sh script
then calls a handler for each of these, the handler is expected to check
for an update, make the necessary APKBUILD changes and commit them.

After all packages have been updated, the changes are pushed to pmaports
master using a project access token.

[ci:skip-build] takes too long to build in CI