[8.x](backport #4169) Refactor API struct constructor

[mergify]

What is the problem this PR solves?

Refactor API constructor to use functional opts instead of a long list of pointers.

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

• Closes cleanup api server struct creation #3823

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  This is an automatic backport of pull request Refactor API struct constructor #4169 done by Mergify.

[elastic-sonarqube]

Quality Gate passed

Issues
1 New issue
1 Fixed issue
0 Accepted issues

Measures
0 Security Hotspots
96.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube