Add CLA signature enforcement bot. (#106)

Sorry, the lawyers insist.

(This is the same bot we use on the workerd repo.)

Author: kentonv

.github/workflows/cla.yml

@@ -0,0 +1,30 @@
+name: "CLA Assistant"
+on:
+  issue_comment:
+    types: [created]
+  pull_request_target:
+    types: [opened,synchronize]
+  merge_group:
+
+permissions:
+  actions: write
+  contents: write # to store signatures on cla-signatures branch
+  pull-requests: write
+  statuses: write
+
+jobs:
+  CLAssistant:
+    runs-on: ubuntu-latest
+    steps:
+      - name: "CLA Assistant"
+        if: (github.event.issue.pull_request && (github.event.comment.body == 'recheck' || github.event.comment.body == 'I have read the CLA Document and I hereby sign the CLA')) || github.event_name == 'pull_request_target'
+        uses: contributor-assistant/[email protected]
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          path-to-signatures: 'signatures/version1/cla.json'
+          path-to-document: 'https://www.cloudflare.com/cla/'
+          # branch should not be protected
+          branch: 'cla-signatures'
+          allowlist: dependabot[bot],workers-devprod
+          lock-pullrequest-aftermerge: false