[WIP] Globally-scoped traits

[grynspan]

[One line description of your change]

Motivation:

[Explain here the context, and why you're making that change. What is the problem you're trying to solve.]

Modifications:

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Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[lpenadiaz]

Hey Team. I'm a Senior Engineering Manager from Datadog and we are working on better supporting Swift Testing with one of our products (Test Optimization). We came across this PR and this feature would make the onboarding of our customers more streamlined if we had global traits available. Is this something that you have in your roadmap to re-take at some point in the near future, also, is there something we can do to help to move this forward? We'd be happy to contribute here if you can provide us some guidance on what is it that you're missing to move this PR from draft to review.

[grynspan]

This PR is experimental. We're still discussing how best to turn this into a real feature (and whether or not we should in the first place.) Thanks for the feedback!