Add NGINX Agent config validation

[dhurley]

Proposed changes

• Improved logging to tell user what line in the agent config file is incorrect.
• Agent now detects if a field in the config is unknown.
• Updated default nginx-agent.conf file.

Example log messages:

2025/04/04 15:18:17 ERROR Failed to load configuration file error="[31:1] unknown field \"unknown_field\""

2025/04/04 15:19:07 ERROR Failed to load configuration file error="[25:11] cannot unmarshal string into Go struct field Config.Command of type int"

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

[john-david3]

Unsure why the host and port has been changed from localhost to this host?

[aphralG]

Github wont allow me to click on the other file for some reason, but some of the test files have fatal as an option and dont have warn. The default also doesnt have warn as an option looking at it, should that be changed ?