Add GHA for custom PR review

[sergejparity]

⚠️ We'll synchronize the merge of this PR with the deprecation of the approval features of processbot (paritytech/parity-processbot#373). Please do not merge it until then.

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This PR enables the configuration in pr-custom-review.yml for the rules discussed in paritytech/pr-custom-review#67. Feel free to comment there for any suggestions or disagreements.

In short: Action will monitor PRs for changes defined in rules and if such detected will request review from defined in rule users/teams and will monitor review status by setting by setting Check reviews GitHub Status Check (needs to be enabled in repository settings) on PR which will pass once all needed approvals received.

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This is how it requests the relevant reviewers

Example output of the job failed by the unfulfilled request for reviewing the 🔒ed line.

[sergejparity]

Check reviews status is failing right now with 404 error, but it is expected behavior. This GHA protecting its configuration against being changed directly in PR by taking into account version residing in master branch. And currently it does not exist in master. It will become operational once this PR will get merged.
Afterwards changes to its configuration should go through the usual approval process to become active.

[joao-paulo-parity]

For the record I'll not be able to deprecate the approval rules on processbot (paritytech/parity-processbot#356) until pr-custom-review is available on Polkadot and Cumulus as well. At the same time I intuit that processbot and pr-custom-review do not block nor obstruct each other.

[sergejparity]

#10968 created to deploy config file for this GHA as this PR cannot proceed without it.

[gavofyork]

On the Polkadot side we will need this as well. There we want to protect the Kusama and Polkadot runtime files. Not just the lib.rs. It should at least include the xcm_config.rs and may some of the files from runtime-common. However, we can define this list when the particular Polkadot pr was opened. These changes should be approved by at least 2 people of group of: @gavofyork @shawntabrizi @rphmeier and me.

I'd alter that to: at least two team leads, one of whom is in the group @gavofyork @shawntabrizi, @rphmeier and @bkchr. I think otherwise it adds too much friction.

Same for the 🔒-lines in Substrate.

[joao-paulo-parity]

In light of

• Add GHA for custom PR review #10951 (comment)
• Add GHA for custom PR review #10951 (review)
• Add GHA for custom PR review #10951 (comment)

I've noticed there hasn't been a consensus on the rules' definitions compared to what's implemented in this PR (which is closer to what we were predicting during paritytech/pr-custom-review#32). #10951 (comment) particularly can be interpreted as divergence from the initial requirements in https://github.com/paritytech/ci_cd/issues/233#issue-1026394650 which, I assumed, would not change; if these requirements would change, since they are related to built-in checks, then not only the rules but also the implementation of pr-custom-review needs to change as well.

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Based on how the discussion went so far and how many parties are involved, I also think it's less productive to have this same back-and-forth on each pull request for each repository. I would prefer if we were to define rules for all repositories upfront first and then only move to the implementation once a consensus has been reached, instead of trying to both discuss and implement at the same time like in this PR.

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Given the arguments above I've suggested to @TriplEight that it would be more efficient to discuss the rules in English terms, in a separate PR, using the following format as an example:

Repository A:

- Any PR needs 2 core-devs approvals

- Lock line changes needs both:
  - 1 approval from @paritytech/locks-review 
  - 1 approval from one of @UserA, @UserB

- Files which need 2 approvals from @UserA, @UserB, @UserC:
  - ./utils/frame/*
  - ./utils/lib.rs

Repository B:
- ...

Repository C:
- ...

I plan to soon propose a separate discussion PR in https://github.com/paritytech/pr-custom-review, so I suggest we move this PR to Draft in the meantime.

pls review again

we've fixed everything