static AccessTokenRequest ofResourceOwnerPassword(String username, String password) {
        return ofResourceOwnerPassword(username, password, null, null);
    }
Creates a new {@link AccessTokenRequest} that is used to request an access token using the resource owner
password credentials, as per
<a href="https://datatracker.ietf.org/doc/html/rfc6749#section-4.3">RFC 6749, Section 4.3</a>.
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/AccessTokenRequest.java