add sbt_update_token_references method and event to the standard

neps/nep-0393.md

@@ -399,6 +399,15 @@ trait SBTRegistry {
 
     /// Similar to `sbt_revoke`, but revokes all `owner`s tokens issued by the caller.
     fn sbt_revoke_by_owner(&mut self, owner: AccountId, burn: bool);
+
+    /// Allows issuer to update token metadata reference and reference_hash.
+    /// * `updates` is a list of triples: (token ID, reference, reference base64-encoded sha256 hash).
+    /// Must emit `token_reference` event.
+    /// Panics if any of the token Ids don't exists.
+    fn sbt_update_token_references(
+        &mut self,
+        updates: Vec<(TokenId, Option<String>, Option<Base64VecU8>)>,
+    );
 }
 ```
 
@@ -460,14 +469,14 @@ type u64 = number;
 type Nep393Event {
   standard: "nep393";
   version: "1.0.0";
-  event: "mint" | "recover" | "renew" | "revoke" | "burn" | "ban" | "soul_transfer" ;
-  data: Mint | Recover | Renew | Revoke | Burn | Ban[] | SoulTransfer;
+  event: "mint" | "recover" | "renew" | "revoke" | "burn" | "ban" | "soul_transfer" | "token_reference" ;
+  data: Mint | Recover | Renew | Revoke | Burn | Ban[] | SoulTransfer | TokenReference;
 }
 
 /// An event emitted by the Registry when new SBT is created.
 type Mint {
   ctr: AccountId;    // SBT Contract minting the tokens
-  tokens: [[AccountId, u64[]]];  // list of pairs (token owner, TokenId[])
+  tokens: (AccountId, u64[])[];  // list of pairs (token owner, TokenId[])
 }
 
 /// An event emitted when a recovery process succeeded to reassign SBTs, usually due to account
@@ -519,6 +528,9 @@ type SoulTransfer {
   from: AccountId;
   to: AccountId;
 }
+
+/// An event emitted when existing token metadata references are updated.
+type TokenReference = u64[];  // list of token ids.
 ```
 
 Whenever a recovery is made in a way that an existing SBT is burned, the `Burn` event MUST be emitted. If `Revoke` burns token then `Burn` event MUST be emitted instead of `Revoke`.