Added GPG support

Author: simasch

.github/workflows/release.yml

@@ -39,6 +39,8 @@ jobs:
           server-id: ossrh
           server-username: MAVEN_USERNAME
           server-password: MAVEN_PASSWORD
+          gpg-private-key: ${{ secrets.MAVEN_GPG_PRIVATE_KEY }}
+          gpg-passphrase: MAVEN_GPG_PASSPHRASE
 
       - name: Cache local Maven repository
         uses: actions/cache@v2
@@ -53,3 +55,4 @@ jobs:
         env:
           MAVEN_USERNAME: ${{ secrets.OSSRH_USERNAME }}
           MAVEN_PASSWORD: ${{ secrets.OSSRH_TOKEN }}
+          MAVEN_GPG_PASSPHRASE: ${{ secrets.MAVEN_GPG_PASSPHRASE }}