Chart: Enable templating in extraContainers and extraInitContainers

[mewa]

Currently, the extraContainers and extraInitContainers are passed on an "as-is" basis. What this means in practice is that if you depend on a release-specific resource you have to manually provide the names rather than use templating.

---

Example: mounting a release-specific ConfigMap into an extraContainer

The following config will not work, since it produces an init container which references {{ .Release.Name }}-stuff-config copied verbatim, rather than the templated value including the actual release name as you would expect.

workers:
  extraInitContainers:
    - name: init-stuff
      image: does/stuff:1.0.0
      envFrom:
        - configMapRef:
            name: "{{ .Release.Name }}-stuff-config"

extraConfigMaps:
  '{{ .Release.Name }}-stuff-config':
    data: |-
      STUFF_ENV_VAR1: one
      STUFF_ENV_VAR2: two

---

This PR enables templating for extraContainers and extraInitContainers fields to fix this issue and improve the reusability of the chart.

---

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[mewa]

Rebased on main

[JuniorJPDJ]

IMO would also be nice to add informations about templating to the comments in values file itself (as in done volumes)

I'm personally waiting for this to be merged as I depend on this feature in umbrella chart I'm writing.