Introduce `apm-server.data_streams.enabled` config,
which will be used by idxmgmt to route events to
data streams based on data_stream.* fields that are
expected to be in each published event.

When transforming model objects into beat.Events,
set the data_stream.{type,dataset} fields. We add
data_stream.namespace elsewhere, using an event
processor.

Handle the new apm-server.data_streams.enabled
config:
 - when enabled, we add data_stream.namespace to
   all published events
 - when disabled, we remove data_stream.* fields
   from all published events

For now we just set data_stream.namespace to "default".
Later this will be based on the config received from Fleet.

There is a hack in here to inject data_stream.namespace
in all published events, since the tests do not use the
standard libbeat pipeline code.

Use a common "apm." prefix, and place the service name last.