add new function ngx_link_func_get_uri

new function to getting URI

sample code response

    ngx_link_func_str_t uri;
    if (ngx_link_func_get_uri(ctx, &uri) == 0) {
        ngx_link_func_write_resp(
            ctx,
            200,
            "200 OK",
            "text/plain",
            uri.data,
            uri.len
        );
    } else {
        ngx_link_func_write_resp(
            ctx,
            404,
            "404 NOT FOUND",
            "text/plain",
            NULL,
            0
        );
    }