Add toString() to SessionKey

pgpainless-core/src/main/java/org/pgpainless/util/SessionKey.java

@@ -7,6 +7,7 @@ package org.pgpainless.util;
 import javax.annotation.Nonnull;
 
 import org.bouncycastle.openpgp.PGPSessionKey;
+import org.bouncycastle.util.encoders.Hex;
 import org.pgpainless.algorithm.SymmetricKeyAlgorithm;
 
 /**
@@ -57,4 +58,9 @@ public class SessionKey {
         System.arraycopy(key, 0, copy, 0, copy.length);
         return copy;
     }
+
+    @Override
+    public String toString() {
+        return "" + getAlgorithm().getAlgorithmId() + ":" + Hex.toHexString(getKey());
+    }
 }