Add support for context variables in --header option

[blakerouse]

Overview

At the moment the --header option only allows as specific key/value pair to be used that doesn't change. Support is needed to allow passing in the ability for the value of the key/value pair to come from a context variable provider.

e.g. --header "${env.ES_HEADER}" or --header "${filesource.source1.proxy_header}"

Why?

These values can change during the life of the running Elastic Agent and new values need to be used to allow the communication to keep occurring.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)