[Fleet] Support inaccessible Elastic Package Registry in air-gapped environments

[kpollich]

Fleet should be able to offer a limited experience around browsing and installing packages even if an instance of EPR isn't accessible on the network.

Context

Existing support for air-gapped environments

Fleet's documentation currently details two approaches to supporting air-gapped environments:

• Using a proxy server to allow access to https://epr.elastic.co
• Self-hosting an instance of the EPR service via Docker

Bundled packages support

In #122297 (parent issue: #112095), we added support for bundling packages along with Kibana to remove Fleet's dependency on EPR during its setup process. We no longer need to have an accessible instance of EPR in order to install packages so long as they are bundled with Kibana. This allows us to preconfigure things like APM and Fleet Server integrations and avoid costly or potentially brittle network calls during Kibana's boot process.

Now that this functionality is in place, we should do a pass of Fleet's logic to account for cases in which EPR is inaccessible, and serve up a limited Fleet/Integrations experience.

Goals

• Fleet Setup should succeed without an accessible EPR instance
• A user should be able to view the "browse integrations" and "installed integrations" screens without an accessible EPR instance
  • Note: These screens won't include bundled packages as "browse-able" resources, but they should be accessible without error even if an EPR service is not reachable
• A user should be able to view the "details" page for their installed integrations without an accessible EPR instance

Current State

Fleet Setup

Currently, if the EPR is inaccessible, Fleet Setup will succeed with a non-fatal error and will not install a package even if it's bundled. e.g.

This should be a relatively quick fix to catch this error and allow bundled install to continue. I believe this error comes from Fleet attempting to query the Registry for the latest version here:

kibana/x-pack/plugins/fleet/server/services/epm/packages/bulk_install_packages.ts

Lines 40 to 45 in 73cc08e

	const packagesResults = await Promise.allSettled(
	packagesToInstall.map((pkg) => {
	if (typeof pkg === 'string') return Registry.fetchFindLatestPackage(pkg);
	return Promise.resolve(pkg);
	})
	);

[elasticmachine]

Pinging @elastic/fleet (Feature:EPM)

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

Closing in favor of #74614