feat: add ChangeColumnType expr (#149)

* feat: add `ModifyColumn`

* style: rename `ModifyColumn` -> `ChangeColumnType`

* style: rename `modify_columns` -> `change_column_types`

* style: remove `ChangeColumnTypes` on `server.proto`

* resolve conflicts

proto/greptime/v1/ddl.proto

@@ -80,6 +80,7 @@ message AlterExpr {
     AddColumns add_columns = 4;
     DropColumns drop_columns = 5;
     RenameTable rename_table = 6;
+    ChangeColumnTypes change_column_types = 7;
   }
 }
 
@@ -115,6 +116,8 @@ message AddColumns { repeated AddColumn add_columns = 1; }
 
 message DropColumns { repeated DropColumn drop_columns = 1; }
 
+message ChangeColumnTypes { repeated ChangeColumnType change_column_types = 1; }
+
 message RenameTable { string new_table_name = 1; }
 
 message AddColumn {
@@ -122,6 +125,12 @@ message AddColumn {
   AddColumnLocation location = 3;
 }
 
+message ChangeColumnType {
+    string column_name = 1;
+    ColumnDataType target_type = 2;
+    ColumnDataTypeExtension target_type_extension = 3;
+}
+
 message DropColumn { string name = 1; }
 
 message TableId { uint32 id = 1; }