[9.0](backport #4290) Add ability to enroll with a specific ID

[mergify]

What is the problem this PR solves?

This solves an issue where an Elastic Agent is being replaced with a new Elastic Agent instance for the same host, pod, or workload. This allows the enrolling Elastic Agent to tell the ID that it wants to use, that ID can be currently in-use and this enrollment will take over the record of that Elastic Agent. To take off the existing Elastic Agent both the original and the new enrollment must use the same replace_token during the enrollment. This ensures that the original enrollment informs Fleet Server that it can be replaced, and ensures that the replacement has the same token to perform the replacement.

How does this PR solve the problem?

It solves the issue by taking a new id field in the enroll HTTP request. That id is then used as the Elastic Agent ID and determines if this is a new Elastic Agent or if it should take over an existing Elastic Agent record.

How to test this PR locally

At the moment the integration tests are the best way to test this, as the ability to use this field has not been exposed yet on the Elastic Agent.

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc. (already covered by enroll handle)

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (no config changes)
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

• Closes Add ability to provide the agent-id in enroll API #4226

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  This is an automatic backport of pull request Add ability to enroll with a specific ID #4290 done by Mergify.

[elastic-sonarqube]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
79.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube