Support non-map format for image pull secrets config

[weyfonk]

(nice to have, idea by @Mauraza)

Image pull secrets can currently only be provided in map format, but we could support multiple formats, e.g.:

# Currently supported:
imagePullSecrets:
  - name: secret-name

# Should ideally be supported too:
imagePullSecrets:
  - secret-name

Fleet's Helm chart templates could provide helpers enabling this, e.g.:

{{- define "imagesPullSecrets" -}}
  {{- $pullSecrets := list }}

  {{- range ((.global).imagePullSecrets) -}}
    {{- if kindIs "map" . -}}
      {{- $pullSecrets = append $pullSecrets .name -}}
    {{- else -}}
      {{- $pullSecrets = append $pullSecrets . -}}
    {{- end }}
  {{- end -}}

  {{- range .imagePullSecrets -}}
    {{- if kindIs "map" . -}}
      {{- $pullSecrets = append $pullSecrets .name -}}
    {{- else -}}
      {{- $pullSecrets = append $pullSecrets . -}}
    {{- end -}}
  {{- end -}}

  {{- if (not (empty $pullSecrets)) -}}
imagePullSecrets:
    {{- range $pullSecrets | uniq }}
  - name: {{ . }}
    {{- end }}
  {{- end }}
{{- end -}}