Github actions coverage

[AndWeHaveAPlan]

Jobs for coverage collections and upload to codecov for github PR's
Uses same test suit as test-linux-stable, splits tests into 5 parallel jobs, uploads to codecov, generates report comment and status checks (can be made required)

Codecov behavior (required coverage, thresholds, comment info etc.) is highly customizable via .github/codecov.yaml (reference)

Unfortunately, some tests are excluded because with -C instrument-coverage they run very slowly, flaky, or fail (see nextest filter expression of test-linux-stable-coverage). So for now, this workflow is optional, and will only run for pr's with the GHA-coverage label

[ordian]

Approving rust changes similar to #2644

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.46%. Comparing base (37b1544) to head (31de062).
Report is 582 commits behind head on master.

❗ Current head 31de062 differs from pull request most recent head 3c09678

Please upload reports for the commit 3c09678 to get more accurate results.

Additional details and impacted files

Files with missing lines	Coverage Δ
substrate/bin/node/runtime/src/lib.rs	10.58% <ø> (ø)
substrate/frame/babe/src/mock.rs	86.44% <ø> (ø)
substrate/frame/grandpa/src/mock.rs	80.92% <ø> (ø)

... and 2693 files with indirect coverage changes

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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