Allow reports to specify a column/metadata other than "label" that uniquely identifies a row

[diosmosis]

Description:

As title. Requested by @tsteur.

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed

[sgiehl]

@diosmosis there seems to be a system test failing. Otherwise guess we could merge that then.

[diosmosis]

@sgiehl relevant test failures should be fixed