Add ability to use go-getter for fetching remote manifests directory as chart

[mumoshu]

This, in combination with #1172, allows you to use go-getter-supported URL for K8s manifests on chart, so that Helmfile automatically fetches it and then turn it into a temporary local chart, which is then installed by Helmfile as similar as standard Helm charts.

An example usecase of this is to install cert-manager CRDs which is distributed separately from the chart:

releases:
- name: cert-manager-crds
  chart: git::http://github.com/jetstack/cert-manager.git@deploy/crds?ref=v0.15.2

I'm adding this based on discussion with @lukasmrtvy. He was trying to install cert-manager and prometheus-opreator with Helmfile. This combined with #1373 should do the job. Thanks for the input!

[osterman]

@mumoshu you're on a roll! this is awesome and fixes a big painpoint for us.

[lukasmrtvy]

@mumoshu

It will probably not work for cert-manager:
https://github.com/jetstack/cert-manager/blob/master/deploy/crds/crd-challenges.yaml#L6, due to internal templating..

Here is my workaround:

  hooks:
  - events: ["presync"]
    command: "/bin/sh"
    showlogs: true
    args: ["-c", "kubectl get --request-timeout='5m' -f https://github.com/jetstack/cert-manager/releases/download/{{`{{ .Release.Version }}`}}/cert-manager.crds.yaml || kubectl apply --request-timeout='5m' --validate=false -f https://github.com/jetstack/cert-manager/releases/download/{{`{{ .Release.Version }}`}}/cert-manager.crds.yaml"] 
  - events: ["postuninstall"]
    command: "/bin/sh"
    showlogs: true
    args: ["-c","kubectl delete --request-timeout='5m' --ignore-not-found -f https://github.com/jetstack/cert-manager/releases/download/{{`{{ .Release.Version }}`}}/cert-manager.crds.yaml"]

cavetas: https://cert-manager.io/docs/installation/upgrading/upgrading-0.15-0.16/#issue-with-older-versions-of-kubectl

Isnt it possible to use something like:

releases:
- name: cert-manager-crds
  chart: file::https://github.com/jetstack/cert-manager/releases/download/v0.16.1/cert-manager.crds.yaml
  forceGoGetter: true

?