Added new extension "sync"

Periodically poll registries and pull images according to sync's config
Added sync on demand, syncing when clients asks for an image which
zot doesn't have.

Signed-off-by: Petu Eusebiu <peusebiu@cisco.com>

examples/sync-auth-filepath.json

@@ -0,0 +1,10 @@
+{
+    "localhost:8080": {
+        "username": "user",
+        "password": "pass"
+    },
+    "registry2:5000": {
+        "username": "user2",
+        "password": "pass2"
+    }
+}