@CanIgnoreReturnValue
    public Builder addPem(String pem, PemKeyType keyType) {
      PemKey pemKey = new PemKey();
      pemKey.reader = new BufferedReader(new StringReader(pem));
      pemKey.type = keyType;
      pemKeys.add(pemKey);
      return this;
    }
Adds a PEM.

<p>A single PEM can contain multiple keys, but all must have the same {@code keyType}.
Invalid or unparsable keys are ignored.

<p>The first key in the first added PEM is the primary key.
java_src/src/main/java/com/google/crypto/tink/signature/SignaturePemKeysetReader.java