Add agent_policy_id and policy_revision_idx to checkin request

[michel-laterman]

What is the problem this PR solves?

Agent policy details (id + revision) may go out of sync with what fleet-server records.
This occurs when a VM running and agent with policy X on revision N is restored to an earlier time when it was running X N-1, or even a different policy. It can also occur if the ES cluster is restored to an earlier snapshot where the agent will be running policy N+1.

How does this PR solve the problem?

Allow the agents to add their currently running policy_id and
revision_idx attributes to the checkin request bodies. These attributes,
if included and different from the agent doc will be used when updating
the agent doc in the pre-poll checkin. If the agent's policy id does not
match the expected policy id from the server a reassign is detected and
a new policy change action will be sent. If the revision differs a policy
change action will also be sent. If an agent checks in with a different
policy/revision the api keys may be managed. Add a feature flag to disable
this behaviour and only use Acks + the fleet-agents doc as the source of
truth.

How to test this PR locally

Added testing in the e2e API test suite, run:

mage test:e2e

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.

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
• 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

• Relates Fleet check-in should send policy_id and revision elastic-agent#6446

[prodsecmachine]

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[michel-laterman]

Integration or E2E tests can be added after elastic/elasticsearch#134517 has been merged and a snapshot with the changes is available.

[elastic-sonarqube]

Quality Gate passed

Issues
4 New issues
1 Fixed issue
0 Accepted issues

Measures
0 Security Hotspots
91.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube

[blakerouse]

Thanks for adding the fix for the revision being higher. Looks good.