[FEATURE] Add support for additional prometheus scrape config

[bsek]

Due to our use of envoy proxies in our cluster we need some custom scrape config in prometheus as the proxies don't get scraped by default.

We have currently worked around this issue by applying the changes described here:
https://github.com/prometheus-operator/prometheus-operator/blob/master/Documentation/additional-scrape-config.md to the CRD provided by okctl.

But it would be nice if the block:

additionalScrapeConfigs: name: additional-scrape-configs key: prometheus-additional.yaml

was already included in the prometheus CRD when we configure a new cluster.