message: Add a convenient function that allows to instantiate a KeysetHandle from a keyset.
diff: @@ -64,6 +64,12 @@ public final class CleartextKeysetHandle {
  public static Keyset getKeyset(KeysetHandle keysetHandle) {
  return keysetHandle.getKeyset();
  }
  +
  + /** Returns a KeysetHandle for {@code keyset}. */
  + public static KeysetHandle fromKeyset(Keyset keyset) throws GeneralSecurityException {
  + return KeysetHandle.fromKeyset(keyset);
  + }
  +
  /**
  * Serializes and writes the {@link Keyset} managed by {@code handle} to {@code keysetWriter}.
  *
  