Allow redis shards discovery without full-fledged k8s dataclient

[AlexanderYastrebov]

Is your feature request related to a problem? Please describe.

#2009 added dynamic discovery of Redis shards via Kubernetes service endpoints.
This implementation relies on k8s dataclient present as it uses the endpoints data that is updated periodically by routing datasource poll cycle.

This works fine for k8s cluster-ingress type of deployment where Skipper needs k8s dataclient anyway.
It is not great for application type deployment where full k8s dataclient is not needed. Most likely users would need to hack configure k8s dataclient and e.g. use a fake selector (#2045) or ingress/routegroup class (#334, #1456) to not fetch Ingresses and RouteGroups from the cluster.

Describe the solution you would like

Skipper supports Redis shards discovery in k8s without fetching other cluster resources.
Ideally k8s dataclient is not registered into the routing to not produce empty/default routes (like https redirect).
Maybe it is possible to use k8s dataclient's cluster client to fetch redis endpoints and not rely on datasource poll cycle.

Describe alternatives you've considered (optional)
A clear and concise description of any alternative solutions or features you've considered.

Additional context (optional)
Add any other context or screenshots about the feature request here.

Would you like to work on it?
Yes

[AlexanderYastrebov]

Skipper also supports pulling endpoints via HTTP (e.g. from routesrv) #2242
Might be nice to support pulling directly from k8s apiserver (with ServiceAccount token).

Users may also deploy routesrv and only use redis endpoints API but that adds another moving part.

[AlexanderYastrebov]

Users may also deploy routesrv and only use redis endpoints API

routesrv may be configured to produce no routes, e.g. to only serve Redis endpoints from the namespace without
Ingresses/RouteGroups:

bin/routesrv \
-enable-kubernetes-endpointslices \
-kubernetes-namespace=my-redis-ns \
-kubernetes-redis-service-namespace=my-redis-ns \
-kubernetes-redis-service-name=my-redis \
-kubernetes-https-redirect=false \
-kubernetes-healthcheck=false \
...