Send upgrade details to Fleet Server in check-in API requests

[ycombinator]

What does this PR do?

This PR sends the current upgrade details to Fleet Server as part of the check-in API's request body.

Why is it important?

To give Fleet users more visibility into the Agent upgrade process.

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

How to test this PR locally

1. Spin up a stack deployment with the latest SNAPSHOT version. As of this writing, that would be version 8.12.0-SNAPSHOT.
2. Build Elastic Agent from this PR's branch but deliberately giving it a version older than the latest SNAPSHOT version.

   AGENT_PACKAGE_VERSION=8.3.0 EXTERNAL=true SNAPSHOT=true PLATFORMS=darwin/arm64 PACKAGES=targz mage package
   

3. Enroll the built Agent with Fleet.
4. Once the Agent shows up in the Fleet UI, upgrade it to the latest SNAPSHOT version available. For this, you will have to manually enter the version in the "Upgrade version" text box in the popup, e.g. 8.12.0-SNAPSHOT.
5. Observe that the Fleet UI shows upgrade details, for example:
   

Related issues

• Relates to Track and report upgrade details #3119 (comment)

[mergify]

This pull request does not have a backport label. Could you fix it @ycombinator? 🙏
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.

[elasticmachine]

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Build stats

• Start Time: 2023-11-14T16:29:50.834+0000

• Duration: 14 min 15 sec

❕ Flaky test report

No test was executed to be analysed.

🤖 GitHub comments

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To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages.

• run integration tests : Run the Elastic Agent Integration tests.

• run end-to-end tests : Generate the packages and run the E2E Tests.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b send-upgrade-details-to-fleet upstream/send-upgrade-details-to-fleet
git merge upstream/main
git push upstream send-upgrade-details-to-fleet

[ycombinator]

Waiting on the next 8.12.0-SNAPSHOT release so we can test this PR here with the fix from elastic/fleet-server#3084.

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b send-upgrade-details-to-fleet upstream/send-upgrade-details-to-fleet
git merge upstream/main
git push upstream send-upgrade-details-to-fleet

[elasticmachine]

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

[pchila]

LGTM

[elastic-sonarqube]

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