ASN.1: Add TAP 5 metadata placeholder (urls in root)

Once TAP 5 is accepted and implemented,	root role metadata will
allow for an optional URL list element for each top-level role.
See TAP 5 for more details.

The placeholders here are commented-out.

Signed-off-by: Sebastien Awwad <sebastien.awwad@gmail.com>

tuf/encoding/metadata_definitions.asn1

@@ -164,6 +164,9 @@ TUFMetadataDefinitions DEFINITIONS AUTOMATIC TAGS ::= BEGIN
     num-keyids          Length,
     keyids              SEQUENCE OF KeyID,
     threshold           Threshold
+    -- Pending TAP 5 approval and implementation:
+    -- num-urls            Length OPTIONAL,
+    -- urls                SEQUENCE OF VisibleString OPTIONAL
   }