refactor: move /pkg/meta/signatures under /pkg/extensions/imagetrust (#1712)

- the size of the binary-minimal becomes 32MB
- "signatures" package is renamed into "imagetrust" and moved under extensions
- if the binary is not built using "imagetrust" tag then the signatures verification will
not be performed

Signed-off-by: Andreea-Lupu <andreealupu1470@yahoo.com>

pkg/common/common.go

@@ -19,6 +19,10 @@ const (
 	clientCertFilename = "client.cert"
 	clientKeyFilename  = "client.key"
 	caCertFilename     = "ca.crt"
+
+	CosignSignature   = "cosign"
+	CosignSigKey      = "dev.cosignproject.cosign/signature"
+	NotationSignature = "notation"
 )
 
 func Contains[T comparable](elems []T, v T) bool {