What does this PR do?

• Makes the file.managed/archive.extracted signature verification support variable backends
• Adds an execution module that allows to sign and verify data using basic asymmetric algorithms

This allows low-level verification without having to rely on GPG or any other external dependency.

If the new execution module is not deemed to fit into core, just introducing the sig_backend changes would still allow custom modules to provide the functionality.

What issues does this PR fix or reference?

Fixes: #66527
Fixes: #66528

Previous Behavior

• File/archive signature verification is only possible using GPG.

New Behavior

• File/archive signature verification supports variable, user-selectable backends.
• A new backend without reliance on GPG supports more low-level verification of asymmetric signatures.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes