Add APM trace configuration to V2AgentManager

[blakerouse]

Proposed commit message

Added a APM to reload.Registry.

Adds the APM to the reload.Registry to allow a reloadable configurator to be set and automatically have Reload called when an APM trace configuration is sent over the Elastic Agent V2 control protocol.

Checklist

• [x] My code follows the style guidelines of this project
• [x] 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
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc. (no changelog as currently not user facing)

Disruptive User Impact

None.

Related issues

• Closes #39606
• Relates https://github.com/elastic/apm-server/issues/11381

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)