[Policy] Allow map variables for policy key

[lucaspouzac]

Is your feature request related to a problem? Please describe.
Actually, it's not possible to use map on key of policy, if we used nginx CRDs. It's ok if we declare manually this on http-snippets, we cannot re-use it on VirtualServerRoute easily.

Describe the solution you'd like
We would like to be able to use a map as a key variable like ${map_(mapName)}

Describe alternatives you've considered
Manual declaration, without CRDs usage.

geo $ip_whitelist {
    default             limited;
    8.8.8.8/32         nolimit;  
    127.0.0.1/32     nolimit; 
}

map $ip_whitelist $limit {
    limited  $binary_remote_addr;
    nolimit  "";
}

limit_req_zone $limit zone=reqWhitelist:10m rate=10r/s;

Additional context
https://github.com/nginxinc/kubernetes-ingress/blob/v3.4.2/pkg/apis/configuration/validation/policy.go#L479

[github-actions]

Hi @lucaspouzac thanks for reporting!

Be sure to check out the docs and the Contributing Guidelines while you wait for a human to take a look at this 🙂

Cheers!