@Override
    public int hashCode() {
        return super.hashCode() * 31 + refreshToken.hashCode();
    }
A request to refresh an access token using a refresh token as per
<a href="https://datatracker.ietf.org/doc/html/rfc6749#section-6">RFC 6749, Section 6</a>.
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/RefreshAccessTokenRequest.java