[Fleet] Add xpack.fleet.isAirGapped flag

[kpollich]

Summary

Closes #173125

Adds a new xpack.fleet.isAirGapped flag to kibana.yml that allow users in air-gapped environments to inform Fleet that it should "fail fast" and skip unnecessary requests that won't be possible as Kibana doesn't have internet access.

This PR also uses the new flag to skip the API request to the product_versions API if the airgapped flag is set to true. There are probably other places we could use this flag in a similar way, but they can be handled separately from this PR.

Checklist

Delete any items that are not applicable to this PR.

• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[github-actions]

Documentation preview:

• ✨ Changed pages

[apmmachine]

🤖 GitHub comments

Expand to view the GitHub comments

Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• /oblt-deploy-serverless : Deploy a serverless Kibana instance using the Observability test environments.
• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[nchaulet]

LGTM 🚀

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 7608496

Metrics [docs]

✅ unchanged

History

• 💔 Build #185719 failed 7c786db

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

cc @kpollich