@CanIgnoreReturnValue
    public Builder addStringClaim(String name, String value) {
      if (!JsonUtil.isValidString(value)) {
        throw new IllegalArgumentException();
      }
      JwtNames.validate(name);
      payload.add(name, new JsonPrimitive(value));
      return this;
    }
Adds a custom claim of type {@code String} to the JWT.
java_src/src/main/java/com/google/crypto/tink/jwt/RawJwt.java