[Meta][Feature] Start the shipper process(es) and configure inputs to write to them when the shipper feature flag is enabled.

[ph]

Dependency on #218

• Start a shipper for each unique destination used by Filebeat or Metricbeat.
• Supervise and monitor the shipper processes.
• Use the shipper as the output destination for Filebeat and Metricbeat
• Collect health information about the shipper

[cmacknz]

Heartbeat needs some special accommodation when implementing this, they always need the Elasticsearch output configuration even when writing events through the shipper.

• Always provide the Elasticsearch output unit configuration to Heartbeat when using the shipper #1476

[jlind23]

@cmacknz @pierrehilbert the PR was merged, shouldn't we close this issue then?

[blakerouse]

@jlind23 Yes sorry forgot to close it.

[jlind23]

No worries, thanks @blakerouse