Use protobuf Enum exclusively for message sent from Client (#2644)

proto/lib/livebook_proto/app_deployment_status.pb.ex

@@ -4,5 +4,5 @@ defmodule LivebookProto.AppDeploymentStatus do
   field :id, 1, type: :string
   field :deployment_group_id, 2, type: :string, json_name: "deploymentGroupId"
   field :version, 3, type: :string
-  field :status, 4, type: :string
+  field :status, 4, type: LivebookProto.AppDeploymentStatusType, enum: true
 end

proto/lib/livebook_proto/app_deployment_status_type.pb.ex

@@ -0,0 +1,8 @@
+defmodule LivebookProto.AppDeploymentStatusType do
+  use Protobuf, enum: true, syntax: :proto3, protoc_gen_elixir_version: "0.12.0"
+
+  field :connecting, 0
+  field :preparing, 1
+  field :available, 2
+  field :deactivated, 99
+end

proto/messages.proto

@@ -159,11 +159,25 @@ message Agent {
   string deployment_group_id = 4;
 }
 
+/**
+ * We're only using Enum in this case because
+ * the Client is the source of the information,
+ * and the Server will always be up-to-date.
+ *
+ * Otherwise, it shouldn't be used.
+ */
+enum AppDeploymentStatusType {
+  connecting = 0;
+  preparing = 1;
+  available = 2;
+  deactivated = 4;
+}
+
 message AppDeploymentStatus {
   string id = 1;
   string deployment_group_id = 2;
   string version = 3;
-  string status = 4;
+  AppDeploymentStatusType status = 4;
 }
 
 message AppDeploymentStatusReport {