Add release action for Gem publishing.

Currently we don't release gem versions to our internal package
registry, and some codebases even subscribe directly to master on
GitHub. This action allows us to publish proper releases as with our
other gems, so we can have better management of breaking changes.

.github/workflows/release.yml

@@ -0,0 +1,22 @@
+name: Publish gem to GitHub package registry
+
+on:
+  push:
+    tags:
+      - v*
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Ruby
+        uses: gocardless/github-actions/actions/ruby-bundle-install@master
+        with:
+          known_hosts: ${{ secrets.KNOWN_HOSTS_GITHUB_KEY }}
+          robot_ssh_key: ${{ secrets.ROBOT_READONLY_SSH_KEY }}
+          token: ${{ secrets.GITHUB_TOKEN }}
+      - name: Publish Gem
+        uses: gocardless/github-actions/actions/publish-internal-gem@master
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}