Exposed created_at to billable members API

Changelog: added
EE: true

doc/api/members.md

@@ -292,7 +292,8 @@ Example response:
     "web_url": "http://192.168.1.8:3000/root",
     "last_activity_on": "2021-01-27",
     "membership_type": "group_member",
-    "removable": true
+    "removable": true,
+    "created_at": "2021-01-03T12:16:02.000Z"
   },
   {
     "id": 2,
@@ -304,7 +305,8 @@ Example response:
     "email": "john@example.com",
     "last_activity_on": "2021-01-25",
     "membership_type": "group_member",
-    "removable": true
+    "removable": true,
+    "created_at": "2021-01-04T18:46:42.000Z"
   },
   {
     "id": 3,
@@ -315,7 +317,8 @@ Example response:
     "web_url": "http://192.168.1.8:3000/root",
     "last_activity_on": "2021-01-20",
     "membership_type": "group_invite",
-    "removable": false
+    "removable": false,
+    "created_at": "2021-01-09T07:12:31.000Z"
   }
 ]
 ```

ee/lib/ee/api/entities/billable_member.rb

@@ -8,6 +8,7 @@ module EE
         expose :last_activity_on
         expose :membership_type
         expose :removable
+        expose :created_at
 
         private
 

ee/spec/lib/ee/api/entities/billable_member_spec.rb

@@ -21,6 +21,10 @@ RSpec.describe ::EE::API::Entities::BillableMember do
     expect(entity_representation[:last_activity_on]).to eq last_activity_on
   end
 
+  it 'exposes the created_at field' do
+    expect(entity_representation[:created_at]).to eq(member.created_at)
+  end
+
   context 'when the user has a public_email assigned' do
     let(:public_email) { 'public@email.com' }