Issue description

Hi,
Now, I see we had support var, var_combination in apisix limit-* plugins yet.
I think we can support a type like function so that we can write some logic as a function in the key.
Then we can get the key from the function.

Environment

• apisix version (cmd: apisix version): 2.10.2
• 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):