feat(sync): sync can include self url in registry.URLs (#1562)

sync now ignores self referencing urls, this will help
in clustering mode where we can have the same config
for multiple zots

closes #1335

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

errors/errors.go

@@ -108,4 +108,5 @@ var (
 	ErrInvalidTruststoreName          = errors.New("signatures: invalid truststore name")
 	ErrInvalidCertificateContent      = errors.New("signatures: invalid certificate content")
 	ErrInvalidStateCookie             = errors.New("auth: state cookie not present or differs from original state")
+	ErrSyncNoURLsLeft                 = errors.New("sync: no valid registry urls left after filtering local ones")
 )