Add deployment_id to deployment payloads

`deployment_id` is included  in the webhook payload for deployments.
This will allow users to identify the deployment directly,
without going through the deployable object.

Changelog: changed

doc/user/project/integrations/webhooks.md

@@ -1396,6 +1396,7 @@ X-Gitlab-Event: Deployment Hook
   "object_kind": "deployment",
   "status": "success",
   "status_changed_at":"2021-04-28 21:50:00 +0200",
+  "deployment_id": 15,
   "deployable_id": 796,
   "deployable_url": "http://10.126.0.2:3000/root/test-deployment-webhooks/-/jobs/796",
   "environment": "staging",

lib/gitlab/data_builder/deployment.rb

@@ -16,6 +16,7 @@ module Gitlab
           object_kind: 'deployment',
           status: deployment.status,
           status_changed_at: status_changed_at,
+          deployment_id: deployment.id,
           deployable_id: deployment.deployable_id,
           deployable_url: deployable_url,
           environment: deployment.environment.name,

spec/lib/gitlab/data_builder/deployment_spec.rb

@@ -27,6 +27,7 @@ RSpec.describe Gitlab::DataBuilder::Deployment do
 
       expect(data[:status]).to eq('failed')
       expect(data[:status_changed_at]).to eq(status_changed_at)
+      expect(data[:deployment_id]).to eq(deployment.id)
       expect(data[:deployable_id]).to eq(deployable.id)
       expect(data[:deployable_url]).to eq(expected_deployable_url)
       expect(data[:environment]).to eq("somewhere")