Add `---` Separator for Additional Envoy Filters

Bug

Description

I've been migrating to operatorless Istio but got this Helm error:

Helm upgrade failed for release istio-system/istiod with chart istiod@1.25.2-bb.4: error while running post render on files: map[string]interface {}(nil): yaml: unmarshal errors:
  line 25: mapping key "apiVersion" already defined at line 2
  line 26: mapping key "kind" already defined at line 3
  line 27: mapping key "metadata" already defined at line 4
  line 29: mapping key "spec" already defined at line 9

Upon further inspection of the rendered manifest, I found that there's no --- separator between additional Envoy filters I was trying to pass. Unlike istio-controlplane's template, istiod's template does not have the --- separator, so adding that separator should fix the error.

BigBang Version

2.54.0