Better support for Helm templates in Kubesec analyzer

Problem to solve

Our kubesec analyzer scans Kubernetes manifests, but cannot scan Helm templates. It is possible to compile those Helm templates and then scan the output, as @theoretick demonstrates here.

We should incorporate this functionality into our kubesec analyzer so that the customer doesn't have to do anything to get this benefit.

Intended users

Further details

From: gitlab-org/security-products/analyzers/kubesec!1 (comment 242132931)

According to https://kubernetes.io/blog/2018/04/24/kubernetes-application-survey-results-2018/ helm is used by 64% of kubernetes users and while it will increase the base size I think it could be quite useful. I'm thinking ideally user shouldn't need the pre-compilation approach and instead move the contents of compile_manifests into a before_script block

Links / references

Release Post: gitlab-com/www-gitlab-com!52655 (merged)

Edited by Taylor McCaslin