Add the ability to customize the sidecar beats configuration

[tanja-milicic]

When enabling stack monitoring, Metricbeat and Filebeat are deployed as sidecar containers within the same Pod as Elasticsearch and Kibana. It would be beneficial to have an easy method for modifying their configurations. For instance, one use case could involve separating audit logs into a distinct index.

[barkbay]

It should be possible to override the configuration as described in #4528 (comment), but I agree, it is certainly not easy and it has significant drawbacks.