[SMTP] Customize Subject Email (by build template)

[michaelact]

Example configuration:

spec:
  notification:
    delivery:
      subject: '{{ if eq .Build.Status 3 }}✅ Build Successful ({{ .Build.Substitutions.REPO_NAME }}){{ else }}❌ Build Failure ({{ .Build.Substitutions.REPO_NAME }}){{ end }} | {{.Build.Substitutions._COMMIT_MESSAGE}}'