Add stackable-generic ServiceMonitor

[maltesander]

Part of stackabletech/issues#747.

Works with all products, http and https.

Based on the prometheus.io/scheme, prometheus.io/port and prometheus.io/path annotations set by the operators.

[dervoeti]

LGTM

[maltesander]

Release snippet

Previously, multiple Prometheus ServiceMonitors were used (e.g. `stackable-hdfs`, `stackable-hbase`) in the Monitoring stack in order to scrape products exposing their metrics differently.
After consolidating the [metrics services](https://github.com/stackabletech/issues/issues/747), one single ServiceMonitor named `stackable` can scrape all Stackable products.