Add more fields to the graphql job_type

What does this MR do?

This adds the duration and finished_at fields to the GraphQL job_type, as part of the upcoming refactor of the jobs page

#320942 (closed)

Screenshots (strongly suggested)

N/A

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content