feat: add plan_artifacts_access to job-templates

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

• feat: add plan_artifacts_access to job-templates

Required when job-templates are used together with the new fetch-mr-plan feature in there, as long as the plan is also coming from the job-templates.

(Still coming from my own fork while waiting for community fork access)