static String getKeyType() {
    return "type.googleapis.com/google.crypto.tink.JwtRsaSsaPkcs1PrivateKey";
  }
This key manager generates new {@code JwtRsaSsaPkcs1PrivateKey} keys and produces new instances
of {@code JwtPublicKeySign}.
java_src/src/main/java/com/google/crypto/tink/jwt/JwtRsaSsaPkcs1SignKeyManager.java