WIP: use go-sysinfo with configurable lower-cased hostname/FQDN

[AndersonQ]

What does this PR do?

Why is it important?

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.md

Author's Checklist

• [ ]

Related issues

• https://github.com/elastic/elastic-agent-system-metrics/pull/177
• https://github.com/elastic/go-sysinfo/pull/236
• https://github.com/elastic/beats/pull/40523
• https://github.com/elastic/beats/pull/40507

[AndersonQ]

not needed anymore