signatures: call notation-go libs instead of invoking notation directly in UTs

[rchincha]

Is your feature request related to a problem? Please describe.

zot invokes "notation" directly in UTs. https://github.com/project-zot/zot/blob/main/pkg/api/controller_test.go#L3457

We can test/assert individual steps.

Describe the solution you'd like

Instead, can we link to this? https://github.com/notaryproject/notation-go

Additional context

https://github.com/notaryproject/notation/pull/141