Enhancement: Manually specify path to *.asc file on disk for airgapped

[cp-elastic]

Describe the enhancement: Add an option to manually specify the path to *tar.gz.asc

Describe a specific use case for the enhancement or feature: Airgapped installations using external automation, such as Ansible.

What is the definition of done? Please add an ability to override the location of the .asc file, or check the --source-uri path if present

Currently it is not possible to securely upgrade Elastic Agent in Airgapped installations due to the behavior of the *tar.gz.asc download. There is no documented option to specify it manually, and it doesn't appear to attempt to look in the --source-uri path, if specified.

This results in an insecure upgrade resulting from having to add the --skip-verify flag. Even manually setting the contents of *tar.gz.asc in the correct path does not seem to work, per the screenshot below.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)