Add ability to exclude project from issue syncing/indexing

[arkid15r]

Some projects have a unique approach for their issues workflows. This means that not all open issues are ready for picking up by potential contributors. For example, https://github.com/OWASP/ASVS/issues project heavily uses issues for discussing the work to be done w/o ever assigning anyone to the issue.

For this type of cases we could use a flag to exclude projects from issues indexing (or even sync) if needed.

This would require introducing a new boolean field for Project model and using it to skip certain actions based on the field's value.

[arkid15r]

As an alternative we could keep them syncing/indexing but just skip AI summary/hint generation step for them.

Open to hear your ideas.