Verify signature for artifacts downloaded during elastic-agent packaging

[pchila]

Describe the enhancement:
Elastic Agent downloads a lot of external archives when creating packages, we should verify the signature of such archives using the relative .asc file.
Describe a specific use case for the enhancement or feature:
Elastic Agent packaging using DRAs or external dependencies
What is the definition of done?
Every downloaded package signature is checked and if any package fails the signature check, packaging must fail

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[elasticmachine]

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