feat(helm): adding helm-diff target

[hjoshi123]

Motivation: cert-manager/cert-manager#8183

CyberArk tracker: VC-46541

[cert-manager-prow]

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[mladen-rusev-cyberark]

/cybr

[inteon]

helm_chart_image_registry probably also needs to be defined.

[hjoshi123]

https://github.com/hjoshi123/makefile-modules/blob/1dcfe9cc44ee5b1240b21962db3da06c64e1378a/modules/helm/helm.mk#L41 I think it is defined already here

[inteon]

Is cert-manager the chart name? could you use a variable for that too?

[hjoshi123]

Oh right.. yes I can do that

[maelvls]

I struggled a little with trying to run this PR from within cert-manager. I found that the helm-diff target isn't reachable from within the cert-manager project:

# From the cert-manager project:
yq -i '(.targets["make/_shared"][] , .targets["make/_shared_new"][]) |= (.repo_ref = "refs/pull/470/head")' klone.yaml
make upgrade-klone

$ make helm-diff
make: *** No rule to make target `helm-diff'.  Stop.

That's because we only import crds.mk in the cert-manager project since we don't use the helm makefile-modules for release the helm chart yet.

In trust-manager, I had the following:

# From the trust-manager project:
$ yq -i '(.targets["make/_shared"][] , .targets["make/_shared_new"][]) |= (.repo_ref = "refs/pull/470/head")' klone.yaml
make upgrade-klone

$ make helm-diff    
make/_shared/helm///helm.mk:193: *** missing separator.  Stop.

Looks like this new target won't work in cert-manager until we are done with cert-manager/cert-manager#7718.

But it will work for all of the other projects such as trust-manager, so I'm in favor of adding it once the above error is fixed.

Great work!