Take care of the authservice image size

[sergicastro]

Authservice could be deployed as a standalone service or as a sidecar next to the app.
The second way to would mean multiple instances of it running in the cluster, in that case, a smaller image is desired.

So we should do something about it, even if it's not using big dependencies or creating different tags including more or less functionalities.

To help on that, I did some investigation on sizes. I detected a big increase between 2 commits by just adding the k8s libs as dependency:

Image or commit	size
ghcr.io/istio-ecosystem/authservice/authservice:0.5.3	~37MB
commit Remove leftover method and improve docs. (#49)	~25MB
commit support loading client secrets from Kubernetes Secret Resource (#26)	~57MB