Add `Unprivileged` state reporting to user-agent, refactor `User-Agent` setup process

[fearful-symmetry]

Proposed commit message

Hopefully the final part of closes https://github.com/elastic/elastic-agent/issues/4683

There's a few things going on here:

• This refactors the way we report config state in the User-Agent, we now put them in the comment field, like so: Elastic-testbeat/8.15.0 (linux; amd64; unknown; 0001-01-01 00:00:00 +0000 UTC; Standalone; Unprivileged)
• This refactors the User-Agent construction process, taking it out of the ES startup and placing it in the beat startup. This centralizes the logic, since nothing in the elasticsearch output constructor needs to actually care about how the User-Agent field is built
• Adds IsUnprivileged() to the User-Agent data
• Adds tests

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] 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.

[mergify[bot]]

This pull request does not have a backport label. If this is a bug or security fix, could you label this PR @fearful-symmetry? 🙏. For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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