feat: Added support for NodePort port numbers configuration

[bidord]

Fixes #28
Relates to fastlorenzo#25

This PR:

• adds the ability to customize the nodePort of each mail service when front.externalService.type=NodePort
• changes the default nodePort numbers to values within the default range 30000-32767

As I commented on #268 (comment), using NodePort numbers < 1024 requires configuring the cluster in a way that is not allowed by many providers.

I think that admins that want to expose mail services on the default port directly on their Kubernetes nodes should use front.hostPort.enabled=true instead. NodePort services should be preferred when using an external load balancer.

My personal Mailu instance is a "2 nodes cluster + NodePort service + haproxy" setup (for now, I have disabled externalService in Helm and added my own Service resource). It took me some digging to configure all of this properly. I started reorganizing my personal notes into a piece of documentation : https://github.com/bidord/mailu-helm-charts/blob/feat-add-documentation-for-nodeport/mailu/README.md#running-behind-an-external-load-balancer-with-a-nodeport-service Tell me if you are interested in me adding it to this PR.

[fastlorenzo]

@bidord thanks for the PR, this looks good! Might be interesting that you add as well the content in the README (could be useful to others as well)

[fastlorenzo]

@bidord could you rebase your code to master? There are some changes in the values files that blocks this merge
Thanks!

[bidord]

@fastlorenzo I just rebased in order to add front.externalService.nodePorts.manageSieve.

I also wanted to modify the documentation part to make it work with latest changes. But I reckon this is still a work in progress? (In master branch, helper function mailu.proxyProtocolPorts is defined but not used anywhere yet.) So for now, I just removed the documentation part from this PR.

Additionally: I think port 465 should be called submissions in values files instead of smtps. This would match better RFC8314 and autoconfig SRV records as provided by Mailu.

[DrPsychick]

@fastlorenzo this also looks good, can we get it merged?