[Elastic Agent] Handle migration of agent_info to agent in Fleet configuration

[blakerouse]

What does this PR do?

#19248 changed the Fleet config to use agent.id instead of agent_info.ID. This was so the configuration could match what Endpoint was expecting.

This PR handles upgrading this transparently on startup for Elastic Agent, so the agent ID will not be re-generated when upgrading to a build that includes #19243

Why is it important?

So the Agent ID is not regenerated when starting Agent after upgrading.

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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

💚 Build Succeeded

Expand to view the summary

Build stats

• Build Cause: [Pull request #19488 updated]

• Start Time: 2020-06-29T13:32:48.263+0000

• Duration: 43 min 30 sec

Test stats 🧪

Test	Results
Failed	0
Passed	543
Skipped	127
Total	670

[blakerouse]

Closing, going to just mark it breaking for 7.8 -> 7.9.