allow file metadata digests to be optional + add link destination

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

schema/json/schema-1.0.4.json

@@ -345,8 +345,7 @@
         "mode",
         "type",
         "userID",
-        "groupID",
-        "digests"
+        "groupID"
       ],
       "properties": {
         "mode": {
@@ -355,6 +354,9 @@
         "type": {
           "type": "string"
         },
+        "linkDestination": {
+          "type": "string"
+        },
         "userID": {
           "type": "integer"
         },