Refactor the bulk checkin handler to use optional args

[michel-laterman]

What is the problem this PR solves?

checkin.Bulk.CheckIn function is hard to extend or understand at a glance.

How does this PR solve the problem?

Refactor the bulk checkin handler to use optional args so the list is easier to read and so that we can more easily add policy ID/revision idx identifiers as a follow up.

How to test this PR locally

mage test

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]

🎉 Snyk checks have passed. No issues have been found so far.

✅ security/snyk check is complete. No issues have been found. (View Details)

✅ license/snyk check is complete. No issues have been found. (View Details)

[michel-laterman]

buildkite run perf-tests

[elastic-sonarqube]

Quality Gate passed

Issues
3 New issues
1 Fixed issue
0 Accepted issues

Measures
0 Security Hotspots
94.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube