Add nginx ingress addon

[berkayoz]

The PR aims to add the nginx ingress controller as an addon.

[neoaggelos]

LGTM, one minor typo

[neoaggelos]

Suggested change

	# unload the the manifests
	# unload the manifests

[OneCricketeer]

Isn't nginx already in microk8s enable ingress?

https://github.com/canonical/microk8s-core-addons/blob/main/addons/ingress/ingress.yaml#L182

[sigv]

Isn't nginx already in microk8s enable ingress?

Current ingress addon references ingress-nginx (the community-maintained variant).
What this PR proposes is adding nginx-ingress variant, owned by the F5 NGINX team themselves.

A notable addition of the F5 NGINX variant is Custom Resource Definitions that allow better readability and more versatility than the generic Ingress resources, for example to define an IP allow-list Policy that is applied to a particular VirtualServerRoute (instead of using annotations on top of annotations for everything).

Blog post reference: A Guide to Choosing an Ingress Controller, Part 4: NGINX Ingress Controller Options.