Add blackbox_exporter deployment

[paulfantom]

What is missing?

For full monitoring flow, we tobs should provide synthetic monitoring capabilities. Since blackbox_exporter is not provided by helm charts from kube-prometheus-stack, we can use experience from kube-prometheus project (which ships blackbox_exporter) and add such functionality.

Why do we need it?

To allow monitoring capabilities from the inside as well as from the outside.

Anything else we need to know?:

[paulfantom]

To consider: do we want to have it enabled by default or not.