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/region/server.proto

@@ -127,6 +127,7 @@ message AlterRequest {
   oneof kind {
     AddColumns add_columns = 2;
     DropColumns drop_columns = 3;
+    ChangeColumnTypes change_column_types = 5;
   }
   // The version of the schema before applying the alteration.
   uint64 schema_version = 4;