Configure RubyGems Trusted Publishing

Based on ruby/net-imap#265

.github/workflows/push_gem.yml

@@ -0,0 +1,48 @@
+name: Publish gem to rubygems.org
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+permissions:
+  contents: read
+
+jobs:
+  push:
+    if: github.repository == 'ruby/net-ftp'
+    runs-on: ubuntu-latest
+
+    environment:
+      name: rubygems.org
+      url: https://rubygems.org/gems/net-ftp
+
+    permissions:
+      contents: write
+      id-token: write
+
+    steps:
+      # Set up
+      - name: Harden Runner
+        uses: step-security/harden-runner@f086349bfa2bd1361f7909c78558e816508cdc10 # v2.8.0
+        with:
+          egress-policy: audit
+
+      - uses: actions/checkout@0ad4b8fadaa221de15dcec353f45205ec38ea70b # v4.1.4
+
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@cacc9f1c0b3f4eb8a16a6bb0ed10897b43b9de49 # v1.176.0
+        with:
+          bundler-cache: true
+          ruby-version: ruby
+
+      # Release
+      - name: Publish to RubyGems
+        uses: rubygems/release-gem@612653d273a73bdae1df8453e090060bb4db5f31 # v1
+
+      - name: Create GitHub release
+        run: |
+          tag_name="$(git describe --tags --abbrev=0)"
+          gh release create "${tag_name}" --verify-tag --draft --generate-notes
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}