feat: add snippet field to memo message (#3689)

proto/api/v1/memo_service.proto

@@ -206,6 +206,9 @@ message Memo {
   // The name of the parent memo.
   // Format: memos/{id}
   optional string parent = 18 [(google.api.field_behavior) = OUTPUT_ONLY];
+
+  // The snippet of the memo content. Plain text only.
+  string snippet = 19;
 }
 
 message MemoProperty {