[chore] Add workflow to check for a linked issue on non-member PRs (#15474)

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#### Description

Adds CI check for PRs from non-members to have a linked issue. This is a
pre-requisite for us to explore implementing other workflows for
flagging duplicate work or work on issues that are not ready to be
worked on.

This tries to start small by excluding PRs marked as `[chore]` and
smalls PRs. We can make it more strict over time.

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#### Authorship

- [x] I, a human, wrote this pull request description myself.

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Author: mx-psi

.github/workflows/check-pr-issue-link.yml

@@ -0,0 +1,43 @@
+name: 'Check PR links to an issue'
+on:
+  pull_request:
+    types:
+      - opened
+      - edited
+      - reopened
+      - ready_for_review
+      - synchronize
+    branches:
+      - main
+
+permissions: read-all
+
+jobs:
+  check-pr-issue-link:
+    runs-on: ubuntu-latest
+    if: >-
+      ${{ github.repository_owner == 'open-telemetry'
+        && github.event.pull_request.user.type != 'Bot'
+        && github.event.pull_request.author_association != 'MEMBER'
+        && !contains(github.event.pull_request.title, '[chore]') }}
+    steps:
+      - name: Check for linked issue
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          REPO: ${{ github.repository }}
+          PR: ${{ github.event.pull_request.number }}
+          ADDITIONS: ${{ github.event.pull_request.additions }}
+          DELETIONS: ${{ github.event.pull_request.deletions }}
+        run: |
+          # Skip trivial PRs, e.g. typo fixes.
+          if (( ${ADDITIONS:-0} + ${DELETIONS:-0} <= 20 )); then
+            echo "PR changes $(( ADDITIONS + DELETIONS )) line(s), trivial, skipping check."
+            exit 0
+          fi
+          COUNT=$(gh pr view "$PR" --repo "$REPO" --json closingIssuesReferences --jq '.closingIssuesReferences | length')
+          if (( COUNT > 0 )); then
+            echo "Found a linked issue."
+            exit 0
+          fi
+          echo '::error::Pull requests from non-members must have a linked issue. Link one from the description, for example "Fixes #1234".'
+          exit 1