Add environmental flags to enable tracing inside container

[blakerouse]

Describe the enhancement:

APM tracing should be able to be enabled for the Elastic Agent container using environment variables. The container supports many different environment variables, but a user is unable to configure the APM tracing through environment variables.

Describe a specific use case for the enhancement or feature:

Allow users to configure APM tracing without requiring mounting a specific configuration to make it work.

What is the definition of done?

When APM tracing works with a set of environment variables.

[elasticmachine]

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