Diagnostics should clearly identify whether an agent is unprivileged or privileged

[cmacknz]

Diagnostics collected from unprivileged agents do not indicate that they are unprivileged. To reproduce:

1. Download the latest 8.14.0-SNAPSHOT and run elastic-agent install --unprivileged
2. Collect diagnostics with elastic-agent diagnostics
3. Extract the diagnostics unzip elastic-agent-diagnostics-2024-03-19T17-40-33Z-00.zip -d diag
4. Observe that the privilege level of the agent does not appear in the diagnostics at all rg --ignore-case priv ./diag.

Describe the enhancement:

• Dump the installation method (docker/rpm/deb/tar.gz) to know how the Elastic Agent was installed
• Dump if feature flags were used in the installation (e.g. --base-path, --unprivileged ).
  • Diagnostics should clearly identify whether an agent is unprivileged or privileged #4439

Follow up tasks:

• Show the info in Fleet UI (in the Elastic Agent view page)

Describe a specific use case for the enhancement or feature:

Better understanding of the Elastic Agent installation via diagnostics.
Better troubleshooting.

What is the definition of done?

• Elastic Agent diagnostics report the installation/packaging method
• Elastic Agent diagnostics report feature flags used at install/enroll

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)