GraphQL: ImageSummary to include information about signature validity

[andaaron]

TBD

[andaaron]

In progress. @Andreea-Lupu to provide a link to the PR.

[andaaron]

Also document how to configure zot for verifying image signatures (example, and .md files).
The initial implementation was already merged in #1344

[andaaron]

Documentation has been merged in #1485