Add IsStandalone() to agent info, report management mode to gRPC

[fearful-symmetry]

What does this PR do?

Note: this requires elastic/beats#39403 to work properly.

This is the second half of the changes needed for #3065.

This involves a number of small changes across a number of files, as it requires changing the AgentInfo() interface to report if the agent is running in standalone mode.

There's no tests for this yet, but you can manually test with https://github.com/elastic/mock-es/

just start the mock ES endpoint with mock-es -metrics 5s, install elastic-agent with all default settings, and check the user-agent in the reported metrics.

Checklist

• My code follows the style guidelines of this project
• 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
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

[mergify]

This pull request does not have a backport label. Could you fix it @fearful-symmetry? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

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

NOTE: backport-skip has been added to this pull request.

[blakerouse]

Looks good.

[jlind23]

buildkite test it

[elasticmachine]

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

[elasticmachine]

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

[elastic-sonarqube]

Quality Gate passed

Issues
1 New issue
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
54.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube