Add processor to apply gdk issue category label

What does this MR do and why?

Applies the GDK issue category based on the checkbox selection in GDK issue description. Prioritizing this work given that EP team will take over GDK issue triaging from Ash, so it will be very helpful if the issue category checkboxes can automate the issue label.

Re: gitlab-org/gitlab-development-kit#1623 (closed).

Expected impact & dry-runs

When user updates their checkbox selections in the GDK issues, the selection will automatically updates the label by applying and/or removing labels.

Note: the MR is a first iteration, which has not added bi-directional syncing, which means, if applying label without updating the checkboxes, the label will be reverted, because the checkboxes are the source of truth. Team members must update the issue description in order to apply the labels.

I can enable syncing from both sources if desired in a later iteration.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review