Add ability pass API Keys as auth headers

[krzko]

Is your feature request related to a problem?

Some vendors require API keys to be passed as headers (curl -H) via POST requests, need to cater for this scenario

ie export LIGHTSTEP_ACCESS_TOKEN='lightstep-access-token: foo'

Describe the solution you'd like

A variable to check that exists, with a an associated format. Pass in to the net_client_post()

Need to remove the opinionated references to /v1/traces and /v1/metrics.

Describe alternatives you've considered

Additional context