request help: support get the key from function in limit-* plugins

[starsz]

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):

[spacewander]

What about allowing user to register their var in

apisix/apisix/core/ctx.lua

Line 147 in 8f0b066

__index = function(t, key)

?

[starsz]

What about allowing user to register their var in

apisix/apisix/core/ctx.lua

Line 147 in 8f0b066

__index = function(t, key)

?

I think support function would be more flexible.
We can write the function like this:

function generate_key(ctx)
  if ctx.var.http_x_consumer == "a" then
       return "a"
  end

  return "default"
end

Then we can limit the specific request with a special header.

[spacewander]

We can register the variable with a custom function, like:

core.ctx.register_var("my_consumer", function (...)
end)

[starsz]

We can register the variable with a custom function, like:

core.ctx.register_var("my_consumer", function (...)
end)

Oh, that's good. Wait for the new feature.