feat(template): added secret template function (#1221)

* feat(tmpl): added fetchSecretValue template function

This adds a tmpl `fetchSecretValue` and `expandSecretRefs` function by:
- Adding:
    - `expandSecretRefs` function in tmpl package that uses vals
    package to fetch secrets
    - `fetchSecretValue` function in tmpl package like below but for
    single string value
    - gomock for tests purpose
- Changing:
    - move init of vals package to function (so the same instance can be used for template values and rendering the whole template)

* doc(secret): added doc how to use new tmpl methods

Added example usage of `fetchSecretValue` and `expandSecretRefs`

docs/remote-secrets.md

@@ -0,0 +1,58 @@
+# Secrets 
+
+helmfile can handle secrets using [helm-secrets](https://github.com/zendesk/helm-secrets) plugin or using remote secrets storage 
+(everything that package [vals](https://github.com/variantdev/vals) can handle vault, AWS SSM etc)  
+This section will describe the second use case. 
+
+# Remote secrets 
+
+This paragraph will describe how to use remote secrets storage (vault, SSM etc) in helmfile 
+
+## Fetching single key
+
+To fetch single key from remote secret storage you can use `fetchSecretValue` template function example below
+
+```yaml 
+# helmfile.yaml 
+
+repositories: 
+  - name: stable 
+    url: https://kubernetes-charts.storage.googleapis.com 
+
+environments: 
+  default: 
+    values:
+      - service:
+          password: ref+vault://svc/#pass
+          login: ref+vault://svc/#login
+releases:
+  - name: service 
+    namespace: default
+    labels:
+      cluster: services
+      secrets: vault
+    chart: stable/svc
+    version: 0.1.0
+    values:
+      - service:
+          login: {{ .Values.service.login | fetchSecretValue }} # this will resolve ref+vault://svc/#pass and fetch secret from vault
+          password: {{ .Values.service.password | fetchSecretValue | quote }}
+      # - values/service.yaml.gotmpl   # alternatively 
+```
+## Fetching multiple keys
+Alternatively you can use `expandSecretRefs` to fetch a map of secrets 
+```yaml
+# values/service.yaml.gotmpl
+service:
+{{ .Values.service | expandSecretRefs | toYaml | nindent 2 }}
+```
+
+This will produce
+```yaml
+# values/service.yaml
+service:
+  login: svc-login # fetched from vault
+  password: pass
+  
+```
+