Add artifacts_file attribute to build data

What does this MR do and why?

Add artifacts_file attribute to build data

• Included artifacts_file attribute in the build method of Gitlab::DataBuilder::Build module.
• The artifacts_file attribute contains filename and size of the build's artifacts.
• Updated build_spec.rb to test the presence and correctness of the artifacts_file attribute.

Why

Job artifacts are an essential part of the event that originates from the Build process. Today, in order for your to understand the artifacts that are generated via API, you must wait until the entire pipeline is completed. This is not an ideal scenario when you are integrating external tools into the process which are dependent on running based on artifact presence.

With the addition of this change, you can now listen to the Job Events and create or perform external jobs as soon as the artifact is available via a job, and no longer wait and/or act as a pipeline blocker.

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