Use GitHub Actions to get signature for Sparkle

Requires a secret called `SPARKLE_ED25519_KEY`, in the format of a private key file

.github/workflows/build.yml

@@ -170,6 +170,27 @@ jobs:
           sudo codesign --sign - --deep --force --entitlements "../program_info/App.entitlements" --options runtime "PolyMC.app/Contents/MacOS/polymc"
           tar -czf ../PolyMC.tar.gz *
 
+      - name: Make Sparkle signature (macOS)
+        if: runner.os == 'macOS'
+        run: |
+          if [ '${{ secrets.SPARKLE_ED25519_KEY }}' != '' ]; then
+            brew install openssl@3
+            echo '${{ secrets.SPARKLE_ED25519_KEY }}' > ed25519-priv.pem
+            signature=$(/usr/local/opt/openssl@3/bin/openssl pkeyutl -sign -rawin -in ${{ github.workspace }}/PolyMC.tar.gz -inkey ed25519-priv.pem | openssl base64 | tr -d \\n)
+            rm ed25519-priv.pem
+            cat >> $GITHUB_STEP_SUMMARY << EOF
+          ### Sparkle Signatures :memo:
+
+          - macOS Archive: $signature
+          EOF
+          else
+            cat >> $GITHUB_STEP_SUMMARY << EOF
+          ### Sparkle Signatures :memo:
+
+          - macOS Archive: No private key available (likely a pull request or fork)
+          EOF
+          fi
+
       - name: Package (Windows)
         if: runner.os == 'Windows'
         shell: msys2 {0}