Add support to set a test run as the baseline

[dgzlopes]

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Feature Description

For people who run k6 tests on CI, having a way to dynamically set a test as the baseline without having to fiddle with our Cloud APIs would be pretty helpful. For example, when you want to establish the baseline to whatever results happened while cutting the latest release of a project.

Suggested Solution (optional)

This could be implemented with some CLI flag:

k6 cloud script.js --cloud.set-as-baseline

Technical details

(I think) Right now, our backend APIs don't let us define a baseline while creating a new test run. We need to do two API calls (Create && set baseline). Maybe this could be improved backend-wise.

Already existing or connected issues / PRs (optional)

No response