Make CI more friendly for external contributors (#4663)

## Problem

CI doesn't work for external contributors (for PRs from forks), see
#2222 for more information.

I'm proposing the following:
- External PR is created
- PR is reviewed so that it doesn't contain any malicious code
- Label `approved-for-ci-run` is added to that PR (by the reviewer)
- A new workflow picks up this label and creates an internal branch from
that PR (the branch name is `ci-run/pr-*`)
- CI is run on the branch, but the results are also propagated to the
PRs check
- We can merge a PR itself if it's green; if not — repeat.

## Summary of changes
- Create `approved-for-ci-run.yml` workflow which handles
`approved-for-ci-run` label
- Trigger `build_and_test.yml` and `neon_extra_builds.yml` workflows on
`ci-run/pr-*` branches

.github/workflows/approved-for-ci-run.yml

@@ -0,0 +1,55 @@
+name: Handle `approved-for-ci-run` label
+# This workflow helps to run CI pipeline for PRs made by external contributors (from forks).
+
+on:
+  pull_request:
+    types:
+      # Default types that triggers a workflow ([1]):
+      # - [1] https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request
+      - opened
+      - synchronize
+      - reopened
+      # Types that we wand to handle in addition to keep labels tidy:
+      - closed
+      # Actual magic happens here:
+      - labeled
+
+env:
+  GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+  PR_NUMBER: ${{ github.event.pull_request.number }}
+
+jobs:
+  remove-label:
+    # Remove `approved-for-ci-run` label if the workflow is triggered by changes in a PR.
+    # The PR should be reviewed and labelled manually again.
+
+    runs-on: [ ubuntu-latest ]
+
+    if: |
+      contains(fromJSON('["opened", "synchronize", "reopened", "closed"]'), github.event.action) &&
+      contains(github.event.pull_request.labels.*.name, 'approved-for-ci-run')
+
+    steps:
+      - run: gh pr --repo "${GITHUB_REPOSITORY}" edit "${PR_NUMBER}" --remove-label "approved-for-ci-run"
+
+  create-branch:
+    # Create a local branch for an `approved-for-ci-run` labelled PR to run CI pipeline in it.
+
+    runs-on: [ ubuntu-latest ]
+
+    if: |
+      github.event.action == 'labeled' &&
+      contains(github.event.pull_request.labels.*.name, 'approved-for-ci-run')
+
+    steps:
+      - run: gh pr --repo "${GITHUB_REPOSITORY}" edit "${PR_NUMBER}" --remove-label "approved-for-ci-run"
+
+      - uses: actions/checkout@v3
+        with:
+          ref: main
+
+      - run: gh pr checkout "${PR_NUMBER}"
+
+      - run: git checkout -b "ci-run/pr-${PR_NUMBER}"
+
+      - run: git push --force origin "ci-run/pr-${PR_NUMBER}"

.github/workflows/build_and_test.yml

@@ -5,6 +5,7 @@ on:
     branches:
       - main
       - release
+      - ci-run/pr-*
   pull_request:
 
 defaults:

.github/workflows/neon_extra_builds.yml

@@ -3,7 +3,8 @@ name: Check neon with extra platform builds
 on:
   push:
     branches:
-    - main
+      - main
+      - ci-run/pr-*
   pull_request:
 
 defaults: