Added filtering jobs by age to jobs/request API endpoint

What does this MR do?

This MR adding optional parameter job_age to API endpoint jobs_request. This parameter is added to allow runners to get jobs only older than job_age what could be used as very simple runner prioritising based on job queue time.

This feature (runner prioritising based on job queue time) is described in gitlab-runner#4177.

This change doesn't brake backward compatibility cause in case of missing job_age parameter jobs are selecting with job_age = 0, what allows to run any job.

Gitlab-Runner MR with corresponding functionality - gitlab-runner!1340 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team