Add support for using a service token to call Kibana Fleet Setup API

[joshdover]

In order to drop the requirement to use superuser credentials to call the Fleet setup API in Kibana, we'd like to switch to using a elastic/fleet-server service account token instead of username & password to authenticate against Kibana.

We should allow specifying a KIBANA_FLEET_SERVICE_TOKEN env var that takes precedence over KIBANA_FLEET_USERNAME and KIBANA_FLEET_PASSWORD. We should also fallback to FLEET_SERVER_SERVICE_TOKEN if provided as well.

I've have a draft branch of what this change may look like, but haven't taken it to completion yet with necessary testing: master...joshdover:fleet-setup-token

This change is targeting 7.16 and is dependent on some work in Kibana (elastic/kibana#112648), however a working draft would help us test this end-to-end before the work is complete on the Kibana side.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)