static OAuth2AuthorizationGrantBuilder builder(WebClient accessTokenEndpoint,
                                                   String accessTokenEndpointPath) {
        requireNonNull(accessTokenEndpoint, "accessTokenEndpoint");
        requireNonNull(accessTokenEndpointPath, "accessTokenEndpointPath");
        return new OAuth2AuthorizationGrantBuilder(accessTokenEndpoint, accessTokenEndpointPath);
    }
Creates a new builder for OAuth 2.0 Access Token Grant flow,
as per <a href="https://datatracker.ietf.org/doc/rfc6749/">[RFC6749]</a>.

@param accessTokenEndpoint A {@link WebClient} to facilitate an Access Token request. Must correspond to
                           the Access Token endpoint of the OAuth 2 system.
@param accessTokenEndpointPath A URI path that corresponds to the Access Token endpoint of the
                               OAuth 2 system.
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/OAuth2AuthorizationGrant.java