enable Sprig functions for embedded Go templates

[qrkourier]

Templating YAML is tedious and error prone, but we can make it less so by introducing the Sprig library (github.com/Masterminds/sprig).

For example, our embedded YAML templates currently have a meaningful number of space characters carefully placed to render with proper indentation. With Sprig, we can instead say "print this string at indentation level 2."

Before:

// note the invisible two spaces preceding "listeners:"
{{ if .Router.IsPrivate }}#{{ end }}  listeners:

After:

{{ if .Router.IsPrivate }}#{{ end }}{{- "listeners:" | nindent 2 }}

[dovholuknf]

personally, i don't see the need for another dependency for this. agree it's annoying but i'm fine with the annoyance to keep yet another dependency out of our code

[qrkourier]

I found another way to make the template slightly less error prone:

{{- if and (not .Router.IsFabric) (eq .Router.TunnelerMode "tproxy") (.Router.Edge.LanInterface) }}
      lanIf: {{ .Router.Edge.LanInterface }}
{{- end }}
{{- if and (not .Router.IsFabric) (eq .Router.TunnelerMode "tproxy") (.Router.Edge.DnsSvcIpRange ) }}
      dnsSvcIpRange: {{ .Router.Edge.DnsSvcIpRange }}
{{- end }}

This way, the conditionally-rendered lines are left-justified, so indentation errors are more visible at a glance.