Add the option to execute a configurable template at each run of HTTP and File target discovery

[karimra]

Suggested by @melkypie in #525
gNMIc HTTP and File target discovery should support prometheus http_sd format.
This would allow gNMIc to use discovery mechanisms available for prometheus.

One way this can be implemented is by adding a template config option under both target discovery mechanisms (http and file) that transforms the Prometheus http_sd format (or any other format) into gNMIc target config format