Add ability to annotate the proxy Service via Helm (gomods#1402)

It is often use to add custom annotations to a Service deployed
through Helm, so this PR adds the ability for a user to specify
annotations in their values file. For such use case we have seen
is when working in conjunction with external-dns, we want to assign
DNS properties of the Service, but currently cannot because this
Helm chart does not support it.

charts/athens-proxy/Chart.yaml

@@ -1,5 +1,5 @@
 name: athens-proxy
-version: 0.3.3
+version: 0.3.4
 appVersion: 0.6.0
 description: The proxy server for Go modules
 icon: https://raw.githubusercontent.com/gomods/athens/master/docs/static/banner.png

charts/athens-proxy/templates/service.yaml

@@ -2,6 +2,10 @@ apiVersion: v1
 kind: Service
 metadata:
   name: {{ template "fullname" . }}
+{{- if .Values.service.annotations }}
+  annotations:
+{{ toYaml .Values.service.annotations | indent 4 }}
+{{- end }}
   labels:
     app: {{ template "fullname" . }}
     chart: "{{ .Chart.Name }}-{{ .Chart.Version }}"

charts/athens-proxy/values.yaml

@@ -11,6 +11,8 @@ image:
   pullPolicy: IfNotPresent
 
 service:
+  ## Additional annotations to apply to the service
+  annotations: {}
   ## Port as exposed by the service
   servicePort: 80
   ## Type of service; valid values are "ClusterIP", "LoadBalancer", and