Github actions coverage (#5148)

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)
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| <img width="640" alt="image"
src="https://github.com/user-attachments/assets/3790f255-27d1-46b9-8d25-2e40b3dcd844">
| <img width="764" alt="image"
src="https://github.com/user-attachments/assets/9e0c8cc7-3278-4b4e-9e28-d76cd57b9163">
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Codecov behavior (required coverage, thresholds, comment info etc.) is
highly customizable via `.github/codecov.yaml`
([reference](https://docs.codecov.com/docs/codecovyml-reference))

Unfortunately, some tests are excluded because with `-C
instrument-coverage` they run very slowly, flaky, or fail (see [nextest
filter
expression](https://github.com/paritytech/polkadot-sdk/pull/5148/files#diff-b19504a9520a2498d03020108344d8e6d93d254d812bfa26247faaa7f55263d6R80)
of test-linux-stable-coverage). So for now, this workflow is optional,
and will only run for pr's with the `GHA-coverage` label