Reintroduce support for fields configuration

[axw]

When we switched to the new ES output in 8.0, we dropped support for adding statically defined fields to all events received by apm-server. Beats has this configuration:

# Optional fields that you can specify to add additional information to the
# output.
#fields:
#  env: staging

This can be used for setting environment-wide fields, such as service.environment or orchestrator.cluster.name. The only practical alternative that I can think of would be to set global labels on all agents feeding into the apm-server in that environment.

We should consider reintroducing this configuration. There would likely need to be two implementations of this -- one for libbeat outputs, and one for the docappender output. For the latter, it can be implemented by using sjson (or similar) after marshalling an event to JSON:

apm-server/internal/beater/processors.go

Lines 96 to 99 in 52cf775

	if err := modeljson.MarshalAPMEvent(event, &r.jsonw); err != nil {
	r.reset()
	return err
	}