public OAuth2AuthorizationGrantBuilder accessTokenRequest(AccessTokenRequest accessTokenRequest) {
        requireNonNull(accessTokenRequest, "accessTokenRequest");
        return accessTokenRequest(() -> accessTokenRequest);
    }
Sets the {@link AccessTokenRequest} to be used when requesting an access token.

<p>Either an {@link AccessTokenRequest} or the {@link Supplier} of an {@link AccessTokenRequest}
must be set before building an {@link OAuth2AuthorizationGrant}.
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/OAuth2AuthorizationGrantBuilder.java