Enforce data streams

As of now, data streams and inputs can always be enabled/disabled via a Kibana toggle switch.

This makes sense for existing integrations, but not so much for APM. Eg., APM records and ingests traces, so if you disable a traces data stream then APM wouldn't work.

We need a way in the spec to define that a data stream is always enabled, so that Kibana doesn't even show a toggle for it.

I suggest a simple boolen attribute `force_enabled` in the data stream `manifest.yml` and default to `false`.