Update Signatures function.

[youssefazrak]

I'm having an issue compiling cosigned using cosign 0.4 or 0.5.
Changes are happening here: https://github.com/sigstore/cosign/blob/main/pkg/cosign/verify.go#L153
Those are leading to the error below in https://github.com/dlorenc/cosigned/blob/main/pkg/cosigned/cosigned.go#L27:

pkg/cosigned/cosigned.go:27:22: not enough arguments in call to cosign.Verify
        have (context.Context, name.Reference, cosign.CheckOpts)
        want (context.Context, name.Reference, *cosign.CheckOpts, string)

pkg/cosigned/cosigned.go:29:3: cannot use key (type *ecdsa.PublicKey) as type cosign.PublicKey in field value:
        *ecdsa.PublicKey does not implement cosign.PublicKey (missing PublicKey method)