Provide custom configuration option per default for all log/metric datasets

Some integrations already offer to configure custom beats configuration options e.g. Kubernetes.

This should be provided as default for all log/metric datasets for all current and future integrations to provide more grain control about beats configuration in background.

E.g. a current usecase:

We have Keycloak instances in Kubernetes and use Keycloak integration to parse the logs. However Keycloak integration cannot collect logs from /var/log/containers as the available files there are symlinks to /var/log/pods.

Via the custom configuration option, it would be possible to add the setting to allow collection from symlinks instead of adding an option for such things to every possible integration.