ts: add metadata field to Idl type (#1422)

ts/src/idl.ts

@@ -12,8 +12,11 @@ export type Idl = {
   events?: IdlEvent[];
   errors?: IdlErrorCode[];
   constants?: IdlConstant[];
+  metadata?: IdlMetadata;
 };
 
+export type IdlMetadata = any;
+
 export type IdlConstant = {
   name: string;
   type: IdlType;