query: sbt_classes and docs

neps/nep-0393.md

@@ -295,6 +295,10 @@ trait SBTRegistry {
     /// If token ID is not found `None` is set in the specific return index.
     fn sbts(&self, issuer: AccountId, token: Vec<TokenId>) -> Vec<Option<Token>>;
 
+    /// Query class ID for each token ID issued by the SBT `issuer`.
+    /// If token ID is not found, `None` is set in the specific return index.
+    fn sbt_classes(&self, issuer: AccountId, tokens: Vec<TokenId>) -> Vec<Option<ClassId>>;
+
     /// Returns total amount of tokens issued by `issuer` SBT contract, including expired
     /// tokens. If a revoke removes a token, it must not be included in the supply.
     fn sbt_supply(&self, issuer: AccountId) -> u64;