Added RFC cron job (#68)

Added RFC Cron job which will iterate over existing referendas and post
a link to them in the open Pull Requests.

This resolves #57

Every time it iterates, it looks for referendas that have been submitted
_after_ the time of the last run, to stop itself from posting duplicated
comments.

It is set to run every day at 12, and it can also be triggered manually.

.github/workflows/rfc-referenda-notifications.yml

@@ -0,0 +1,25 @@
+name: RFC Cron
+
+on:
+    workflow_dispatch:
+    schedule:
+      - cron: '* 11 * * *'
+
+jobs:
+  test_run:
+    runs-on: ubuntu-latest
+    steps:
+        - name: Get last run
+          run: |
+            last=$(gh run list -w "$WORKFLOW" --json startedAt,status -q 'map(select(.status == "completed"))[0].startedAt')
+            echo "last=$last" >> "$GITHUB_OUTPUT"
+          id: date
+          env: 
+           GH_TOKEN: ${{ github.token }}
+           WORKFLOW: ${{ github.workflow }}
+           GH_REPO: "${{ github.repository_owner }}/${{ github.event.repository.name }}"
+        - uses: paritytech/[email protected]
+          env:
+            GH_TOKEN: ${{ github.token }}
+            PROVIDER_URL: "wss://polkadot-collectives-rpc.polkadot.io"
+            START_DATE: ${{ steps.date.outputs.last }}