feat(google): fetch groups in parallel

[codrutpanea]

Overview

What this PR does / why we need it

Fetches nested (transitive) group membership in parallel (with a concurrency limit) instead of strictly sequential API calls, so large/deep group graphs complete faster.

In my tests the loading times went form 10-15 seconds down to 2-3 seconds. I tested the changes with 20 nested groups. And I'm using this fork for over a month with no problems.

Closes #3929

Special notes for your reviewer

[nabokihms]

Thanks for tackling this. Direction is right, but I'd like a few things addressed before merging.

[codrutpanea]

I implemented the suggestions. I also deployed them in my environment to test the changes. Thanks for the feedback.
Also feel free to change anything in this PR if you think it will be faster and easier to merge it.

[nabokihms]

My feedback is mostly addressed - errgroup, ctx, config knob, dead param gone. Nice.
One blocker: recursive g.Go under SetLimit deadlocks. Need a regression test for it.

Other minor stuff inline. Also please squash to one commit and rename to feat(google): - sort order changed and there's a new config field, not really a refactor anymore (and do not forget to sign the commit, otherwise we cannot merge this).

[codrutpanea]

I think i addressed all the feedback, thank you