devops: balance Playwright shards by duration

[aslushnikov]

Right now, one of the shards takes ~25 minutes to complete, whereas
others take 10-15 minutes.

https://github.com/WordPress/gutenberg/actions/runs/28085764857?attempt=1&check_suite_focus=true

This patch will shard tests by their historic duration, which should
even out shards.

Balanced sharding documentation: https://github.com/flakiness/playwright#balanced-sharding

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[aslushnikov]

@Mamaduka The shard balancing algorithm works very well on our tests, and I'm eager to see how it'll perform for your suite.

Could you please take a look?

[aslushnikov]

Oops, for the prs-from-forks, we can't authorize at the flakiness.io service to fetch historical durations. As a result, all bots fail.

That's a bummer! We'll discuss how to accomodate this usecase with the team.

@Mamaduka I've converted this to draft while we're figuring this out.