On week improvements

[michel-laterman]

What is the problem this PR solves?

This PR started in an attempt to add some comments/simplify the fleet-server startup; but has. turned into some minor refactoring/cleanup.

• Add comments and minor restructuring of server/agent.go and server/fleet.go.
• Refactor API server creation.
• Remove most uses of context.Background and context.TODO.

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

[mergify]

This pull request does not have a backport label. Could you fix it @michel-laterman? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[elastic-sonarqube]

Quality Gate passed

Issues
5 New issues
3 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
77.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube