feat: introduce SyncColumns (#261)

proto/greptime/v1/region/server.proto

@@ -166,11 +166,16 @@ message AlterRequest {
     SetIndexes set_indexes = 13;
     UnsetIndexes unset_indexes = 14;
     SetDefaults set_defaults = 15;
+    SyncColumns sync_columns = 16;
   }
   // The version of the schema before applying the alteration.
   uint64 schema_version = 4;
 }
 
+message SyncColumns {
+  repeated RegionColumnDef column_defs = 1;
+}
+
 message AddColumns {
   repeated AddColumn add_columns = 1;
 }