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>

pkg/extensions/config/config.go

@@ -0,0 +1,22 @@
+package config
+
+import (
+	"time"
+
+	"github.com/anuvu/zot/pkg/extensions/sync"
+)
+
+type ExtensionConfig struct {
+	Search *SearchConfig
+	Sync   *sync.Config
+}
+
+type SearchConfig struct {
+	// CVE search
+	CVE    *CVEConfig
+	Enable bool
+}
+
+type CVEConfig struct {
+	UpdateInterval time.Duration // should be 2 hours or more, if not specified default be kept as 24 hours
+}