Refactor Job Artifact report file type constants

What does this MR do and why?

Refactor Job Artifact report file type constants.

This MR combines the various report type file types into one single constant, to be retrieved using a class method. This will allow us to parameterize a report type when necessary, such as in Reports::Factory, instead of having to know the individual constants. This will be used in !93651 (closed)

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