fedcm-example-app: add GitHub Actions workflow to deploy to Heroku on main

Summary:
Adds `.github/workflows/main.yml` so that pushes to `main` on the public mirror at github.com/fbsamples/fedcm-example-app automatically deploy to the `fedcm` Heroku app via the `akhileshns/heroku-deploy` action. Mirrors the workflow added for fbrell in D104124307 (with the SHA fixes from D104458995 already applied — `actions/checkout` pinned to v6.0.2, `heroku-deploy` pinned to v3.15.15).

Note that this file lives internally at `fbcode/third_party_login/fbsamples/fedcm/.github/workflows/main.yml` and ShipIt syncs it to repo root in the OSS mirror.

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overriding_review_checks_triggers_an_audit_and_retroactive_review
Oncall Short Name: third_party_login

Differential Revision: D105223810

fbshipit-source-id: 000775b6083b1287014593225b13a8bede06f24e

Author: mdjasper

.github/workflows/main.yml

@@ -0,0 +1,20 @@
+name: Deploy
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - name: Install Heroku CLI # <- IMPORTANT!!! Make sure the cli is installed before using the action
+        run: |
+          curl https://cli-assets.heroku.com/install.sh | sh
+      - uses: akhileshns/heroku-deploy@1b080913896a5d6d44a18e9e208f8eb1fee9b0f7 # v3.15.15
+        with:
+          heroku_api_key: ${{secrets.HEROKU_API_KEY}}
+          heroku_app_name: "fedcm" #Must be unique in Heroku
+          heroku_email: ${{secrets.HEROKU_EMAIL}}