Issue description

I think the apisix limit-req plugin needs to support combined conditions to limit the request frequency and support whitelisting; just like the tengine limit-req module, it supports multiple conditional restrictions, such as: http_x_forward & http_user_agent are used to limit the request frequency and support white names.
This is the module documentation of tengine limit-req
https://tengine.taobao.org/document_cn/http_limit_req_cn.html

Environment

• apisix version (cmd: apisix version):
• OS (cmd: uname -a):
• OpenResty / Nginx version (cmd: nginx -V or openresty -V):
• etcd version, if have (cmd: run curl http://127.0.0.1:9090/v1/server_info to get the info from server-info API):
• apisix-dashboard version, if have:
• the plugin runner version, if the issue is about a plugin runner (cmd: depended on the kind of runner):
• luarocks version, if the issue is about installation (cmd: luarocks --version):