Add allow_failure attribute to Job API

What does this MR do?

It adds allow_failure attribute to the Job entity serialized for API.

It will be shown for:

• GET /projects/:id/jobs
• GET /projects/:id/jobs/:job_id
• GET /projects/:id/pipelines/:pipeline_id/jobs

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50813

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Style guides