Prepare search rpc & ingress template

No change in staging/production just yet. It's only the templating to prepare this ^.

As an oustanding change from recent template development, this creates 3 functions to ease the rpc-ingress, rpc-autoscale & rpc-service deployment. Instead of duplicating yaml all around. It's possible as those are relatively simple and never diverging (i'll refactor previous templates when this one is merged, the invariant be no changes in swh-helm-diff target).

The deployment (& configmap) parts were kept out of the loop as those differ from one deployment to another (it'd be possible too but more work so not yet).

Usual tests within minikube and template tests are ok.

Refs. swh/infra/sysadm-environment#4780 (closed)