[Fleet] Support version overriding for bundled APM package

[kpollich]

Ref elastic/apm-server#8491

The APM team would like to continuously publish updates to the APM package in the new Package Storage v2 architecture. To support this, Fleet's bundled package process should be able to handle the following:

• Fetch the latest compatible version of the apm package, which may be a prerelease (e.g. provide ?experimental=true in the EPR request)
• Unpack the apm package, update its manifest.yml -> version field to be the same as the current Kibana version, and re-pack it as a zip on disk
• Ensure package verification doesn't re-fire for the APM package bundled on disk, as it will potentially fail when the version is changed

[elasticmachine]

Pinging @elastic/fleet (Feature:EPM)

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[jen-huang]

@kpollich Is it the end user who wishes to upgrade their APM package that is manually executing a bundling process, or are you suggesting for Kibana to execute it at run time?

[kpollich]

@jen-huang - I think we're looking to do this at build-time when Kibana fetches bundled packages. So the workflow would be

1. Kibana build starts
2. "Bundle fleet packages" task is kicked off
3. APM package is designated with some flag in fleet_packages.json, e.g. force_stack_aligned_version: true
4. When bundled packages task fetches APM package, it performs the unpack -> version update -> repack workflow detailed above