public OAuth2AuthorizationGrantBuilder responseHandler(
            OAuth2ResponseHandler<GrantedOAuth2AccessToken> responseHandler) {
        this.responseHandler = requireNonNull(responseHandler, "responseHandler");
        return this;
    }
Sets a custom {@link OAuth2ResponseHandler} to handle the response of an access token request.
If not set, the default response handler will be used.
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/OAuth2AuthorizationGrantBuilder.java