tiflash give "FieldString is not supported" error

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```
drop table if exists tbl_2, tbl_7, tbl_8, tbl_9, tbl_11, tbl_14, tbl_15, tbl_16, tbl_20, tbl_21;
create table tbl_20 ( col_115 boolean not null ,col_116 float not null default 2862.5671249511674 ,col_117 double not null default 433.4597649969197 ,col_118 time ,col_119 varchar ( 100 ) default 'EsgJ9j@*quHqpmAwp' ,col_120 text ( 186 ) not null ,col_121 tinyint ,col_122 mediumint not null default -3999123 ,col_123 varchar ( 326 ) ,col_124 tinyint not null , key idx_35 ( col_119 ( 2 ) ,col_121 ,col_118 ) ) charset utf8mb4 collate utf8mb4_bin partition by hash ( col_122 ) partitions 4;
create table tbl_8 ( col_40 varchar ( 458 ) collate utf8mb4_bin not null ,col_41 float ,col_42 varchar ( 111 ) collate utf8mb4_bin not null default 'iFN1*3sU' ,col_43 mediumint not null default 3553140 ,col_44 time not null , col_45 bigint not null ,key idx_9 ( col_45 ,col_43 ) ) charset utf8mb4 collate utf8mb4_bin partition by hash ( col_45 ) partitions 4;
alter table tbl_20 set tiflash replica 1;
alter table tbl_8 set tiflash replica 1;
set tidb_isolation_read_engines = 'tiflash';
select /*+ hash_agg() */ /*+ hash_join( tbl_20 , tbl_8 */ field( tbl_20.col_120 , tbl_20. col_120 , tbl_20.col_123 ) as r0 , tbl_20.col_117 as r1 from tbl_20 left join tbl_8 on tbl_20.col_121 = tbl_8.col_43 order by r0,r1 limit 64 ;
```

### 2. What did you expect to see? (Required)
no error
### 3. What did you see instead (Required)
[ERROR] [DAGDriver.cpp:202] ["[FLASH:Coprocessor:Unimplemented] FieldString is not supported.

### 4. What is your TiDB version? (Required)
```
MySQL [test]> select tidb_version();
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v6.7.0-alpha
Edition: Community
Git Commit Hash: 145b7cdf72feeecf7f2b0b3e01693c603712dce5
Git Branch: heads/refs/tags/v6.7.0-alpha
UTC Build Time: 2023-03-11 14:37:20
GoVersion: go1.20.2
Race Enabled: false
TiKV Min Version: 6.2.0-alpha
Check Table Before Drop: false
Store: tikv |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.001 sec)
```