feat: Add required parameter to all action inputs (deliverybot#33)

Adds the required parameter to all action.yml input parameters. This is required
by GitHub.

action.yml

@@ -6,29 +6,39 @@ color: gray-dark
 inputs:
   release:
     description: Helm release name. Will be combined with track if set. (required)
+    required: true
   namespace:
     description: Kubernetes namespace name. (required)
+    required: true
   chart:
     description: Helm chart path. If set to "app" this will use the built in helm
       chart found in this repository. (required)
+    required: true
   values:
     description: Helm chart values, expected to be a YAML or JSON string.
+    required: false
   dry-run:
     description: Task name. If the task is "remove" it will remove the configured
       helm release.
+    required: false
   helm:
     description: Helm binary to execute, one of [helm, helm3].
+    required: false
   token:
     description: Github repository token. If included and the event is a deployment
       the deployment_status event will be fired.
+    required: false
   value-files:
     description: Additional value files to apply to the helm chart. Expects JSON encoded
       array or a string.
+    required: false
   secrets:
     description: Secret variables to include in value file interpolation. Expects
       JSON encoded map.
+    required: false
   version:
     description: Version of the app, usually commit sha works here.
+    required: false
 runs:
   using: docker
   image: Dockerfile