Add ability to define the specific agent-id during enroll #6361
Description
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.