@CanIgnoreReturnValue
    public Builder expectIssuer(String value) {
      if (value == null) {
        throw new NullPointerException("issuer cannot be null");
      }
      this.expectedIssuer = Optional.of(value);
      return this;
    }
Sets the expected issuer claim of the token. When this is set, all tokens with missing or
different {@code iss} claims are rejected. When this is not set, all token that have a {@code
iss} claim are rejected. So this must be set for token that have a {@code iss} claim.

<p>If you want to ignore the issuer claim or if you want to validate it yourself, use
ignoreIssuer().

<p>https://tools.ietf.org/html/rfc7519#section-4.1.1
java_src/src/main/java/com/google/crypto/tink/jwt/JwtValidator.java