Add gpg key

Author: laurynas

.github/workflows/publish.yml

@@ -15,6 +15,8 @@ jobs:
           server-id: ossrh
           server-username: MAVEN_USERNAME
           server-password: MAVEN_PASSWORD
+          gpg-private-key: ${{ secrets.GPG_PRIVATE_KEY }}
+          gpg-passphrase: ${{ secrets.GPG_PASSPHRASE }}
       - name: Publish package
         run: mvn --batch-mode deploy
         env: