Implement signed images verification

[konstantina-gramatova]

All used container images must undergo a signature verification (if signed) based on the provided verification data. If the verification fails, running a container instance using such an image must be aborted with the appropriate error.

The verification has to be implemented integrating sigstore's Cosign.
If a global daemon's configuration is applicable, it has to be covered as well.

[dimitar-dimitrow]

Signatures would be pulled only on creation and would be deleted with their corresponding container images. Keys for verification could be provided globally - through container-management configuration, per container - through container configuration. Per container keys are prioritized.

Image Signed	Global Keys	Per Container Keys	No Keys
No	Skip verification	Skip verification	Skip verification
Yes	Perform Verification	Perform Verification	Skip verification

Skip verification - no verification is performed, container is created/started
Perform Verification - if verification fails, container is not created/started; if verification is successful, container is created/started.