Allow reading apm config from file in ESS deployments

[pchila]

What does this PR do?

This PR preserves the agent.monitoring.apm.* settings when the agent is managed and in fleet bootstrap mode (as it is in an ESS deployment)

• PatchAPMConfig is initialized before fleet config is merged onto the file config.
• MonitoringAPMConfig is preserved when loading the persistent configuration
• elastic-agent.yml is not rotated if it already contains fleet.enabled=true (similar to what has been done in Add disable providers by default option #4166, although I didn't manage to make it work on the 8.16 branch where I tested spinning up ESS deployments with custom agent images) so I implemented a simpler solution here
• There's a bunch of logs that need to print only in debug or removed altogether (to figure out what was going on I had to dump part of the configuration in multiple places)

Why is it important?

Allow elastic-agent running fleet-server and apm-server in ESS integration server to propagate APM settings coming from elastic-agent.yml without the need of replicating such configuration in Fleet Cloud policy.

Checklist

• My code follows the style guidelines of this project
• 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
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

[mergify]

This pull request does not have a backport label. Could you fix it @pchila? 🙏
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-v8.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
2 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
42.9% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube