Add ability to define the specific agent-id during enroll

[blakerouse]

Describe the enhancement:

A new flag --agent-id should be added to the enroll command to allow the Elastic Agent to define the ID it wants to use when enrolling with Fleet Server. An ELASTIC_AGENT_ID container environment variable also needs to work the same way as the --agent-id on the enroll command.

Describe a specific use case for the enhancement or feature:

This allows an Elastic Agent to tell Fleet Server that this Elastic Agent is this specific agent.

Most use-cases are described in the sibling issue on fleet-server: elastic/fleet-server#4226

What is the definition of done?

When --agent-id and ELASTIC_AGENT_ID will enroll to the Fleet Server with the provided ID.

[elasticmachine]

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