Support helm release upgrade on helm chart name change

[MichaelMorrisEst]

Summary

Closes #870

Proposes to add a new feature gate UninstallOnChartNameChange.

When enabled (default), the current behaviour of uninstalling a Helm Release when the Helm chart name is changed shall remain unchanged. This results in a new Helm Release install.
When disabled, the Helm Release will not be uninstalled when the Helm chart name is changed. This results in a Helm upgrade being performed. This was the behaviour before Flux 2.2.0

This re-introduces support that was lost going from Flux 2.1.x -> 2.2.x but controls the behaviour through a feature gate so either the previous or current behaviour (default) can be achieved

Changes

• Define UninstallOnChartNameChange feature gate in internal/features/features.go
• Wire the feature gate from main.go to HelmReleaseReconciler
• When the flag is false and the chart name has changed do not uninstall the HelmRelease
• Change the visibility of action.targetChartName to enable it to be referenced in the HelmRelease controller
• Test added for verify new behaviour

[hiddeco]

Feedback beyond the contents of the PR: I do wonder if there are not other changes (e.g., release name/namespace or storage namespace) where there could be a desire of control. Given this, I do wonder if what this PR attempts to do isn't too narrow.

[hiddeco]

The chance of this going into a patch release is unlikely, this should thus indicate 1.6.0.

[hiddeco]

This list of constants was always meant to be a descriptive reason (factored out to a const for code readability), not an identifier. Additionally, only exposing one of them creates a weak contract. I would prefer a solution where:

1. All identifiers are public
2. Identifiers do not contain spaces

[hiddeco]

Some additional thoughts:

• For storage namespace changes, we could actually do the more intelligent thing and attempt to migrate instead of going through an uninstall cycle. This would open a surface for "Secret smuggling" however, and integrity would likely have to be verified as part of this.
• In hindsight, the biggest design mistake I ever introduced in the helm-controller (and something that still bites me as an operator once in a while) is not having the storageNamespace field mirror the releaseNamespace unless explicitly overwritten. If we are rethinking how this all works, this might be worth addressing as well (behind a feature gate).
• Some of these changes may actually require an uninstall (or juggling with Helm install options). As changing a release name would make the resources stay put, which could then cause the subsequent Helm install to fail.