Support enabling mimalloc and turn it on by default

[sslavic]

This was supported in https://github.com/kubernetes/ingress-nginx/releases/tag/nginx-0.26.0 and recently in upstream chart too. We should turn it on to reduce memory utilization - expected at least 10% reduction. Memory utilization increased with recent tuning of max-worker-connections #50 optimization for high performance scenarios.

Even without max-worker-connections switched to 0, some tenant clusters where HPA trials were run had large amount of Ingress resource definitions, resulting in high memory utilization especially during nginx configuration reloads. To better support these use cases out of the box, together with enabling HPA default memory requests for most non-trivial clusters were increased to 2.5GB. Hopefully with applying mimalloc enabled we can lower the utilization. Smaller nginx memory footprint is good for both cost efficiency, faster scheduling and better elasticity - in production use cases like flash sale spikes nginx can scale out / add same capacity faster while creating less pressure on worker nodes scale; with larger nginx it is more likely that additional worker nodes have to be added to place new nginx replicas, and it takes time for cluster-autoscaler to kick in and nodes to be provisioned.

In addition to making mimalloc optional and enabled by default, this PR adds support for configuring additional environment variables for the nginx controller container, like in upstream chart. This among other things enables configuring additional mimalloc options if we find needed.

[sslavic]

This depends on kubernetes/ingress-nginx#5357 being merged and released.

[sslavic]

mimalloc got enabled by default upstream. We've synced defaults with upstream via upgrade to ingress-nginx v0.40.1 PR #122. Closing this PR in favor of the upgrade one.