core: move transaction Attribute value to a designated interface

Signed-off-by: Ekaterina Pavlova <ekt@morphbits.io>

pkg/core/transaction/attribute.go

@@ -8,17 +8,20 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/io"
 )
 
-// Attribute represents a Transaction attribute.
-type Attribute struct {
-	Type  AttrType
-	Value interface {
+// AttrValue represents a Transaction Attribute value.
+type AttrValue interface {
 	io.Serializable
 	// toJSONMap is used for embedded json struct marshalling.
 	// Anonymous interface fields are not considered anonymous by
 	// json lib and marshaling Value together with type makes code
 	// harder to follow.
 	toJSONMap(map[string]any)
-	}
+}
+
+// Attribute represents a Transaction attribute.
+type Attribute struct {
+	Type  AttrType
+	Value AttrValue
 }
 
 // attrJSON is used for JSON I/O of Attribute.