[Fleet] Download Elastic GPG key during build

[hop-dev]

Summary

Part of #133822.

Add a build step to add the Elastic GPG key from https://artifacts.elastic.co/GPG-KEY-elasticsearch to the kibana repo at x-pack/plugins/fleet/target/keys.

The location of the key can be overridden by users with the xpack.fleet.packageVerification.gpgKeyPath config item.

Add a function getGpgKeyOrUndefined in the fleet plugin to access the key, in the interest of keeping the PR's small this method isn't currently called but will be in subsequent verification work for 8.4.

Checklist

Delete any items that are not applicable to this PR.

• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 1e2f48d
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• Webpack Bundle Analyzer

Metrics [docs]

Public APIs missing comments

Total count of every public API that lacks a comment. Target amount is 0. Run node scripts/build_api_docs --plugin [yourplugin] --stats comments for more detailed information.

id	before	after	diff
fleet	1308	1309	+1

Unknown metric groups

API count

id	before	after	diff
fleet	1435	1436	+1

History

• 💚 Build #52462 succeeded 3ec07debf1132b25c16f35ef53366f3d8a40d765
• 💚 Build #52348 succeeded 88ff723d4a50ce22e8bd85341caa4ee0612ec9c3
• 💚 Build #52244 succeeded a84221a5d34b20b93c6794c2f0ead3c66606f35f

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[kpollich]

🚀