Decide what the default shipper Elasticsearch output configuration should be

[cmacknz]

The Elasticsearch output in the shipper is not the same as Beats and is configured differently. We need to decide what the default configuration should be when no other output parameters have been configured.

The current defaults configured in the code are:

elastic-agent-shipper/output/elasticsearch/config.go

Lines 47 to 54 in a526fd6

	// Defaults to runtime.NumCPU()
	NumWorkers int `config:"num_workers"`
	// Defaults to 5MB
	BatchSize int `config:"batch_size"`
	// Defaults to 30sec.
	FlushTimeout time.Duration `config:"flush_timeout"`

We should decide if these are the correct defaults before the shipper beta.

We should also ensure the defaults in the code match the defaults in the reference configuration:

elastic-agent-shipper/elastic-agent-shipper.yml

Lines 193 to 195 in a526fd6

	#num_workers: 3
	#batch_size: 10000000
	#flush_timeout: 30s