public static Function<? super HttpClient, OAuth2Client> newDecorator(
            OAuth2AuthorizationGrant authorizationGrant) {
        requireNonNull(authorizationGrant, "authorizationGrant");
        return delegate -> new OAuth2Client(delegate, authorizationGrant);
    }
Creates a new {@link HttpClient} decorator that handles OAuth 2.0 Authorization Grant flow.
@param authorizationGrant An {@link OAuth2AuthorizationGrant} implementing specific
                          OAuth 2.0 Authorization Grant flow.
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/OAuth2Client.java