This change adds the ability to use substring substitution to inject
numeric source values into string targets. This is required for our
networking catalogue, since a numeric k8s API port needs to be inserted into
numeric and (sub)string targets in different phases.
Co-Authored-By: Ian Howell <ian.howell0@gmail.com>
Change-Id: I24beb46a2bda4e118406129a0a922b0c56142c76