Correlation column has function processing and supports index join

## Feature Request

**Is your feature request related to a problem? Please describe:**

Indexed column cannot use Index Join when it undergoes function processing.
```
mysql> show create table test;
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| test | CREATE TABLE `test` (
`id` bigint(20) DEFAULT NULL,
`dt` datetime DEFAULT NULL,
KEY `idx_dt` (`dt`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select version();
+--------------------+
| version() |
+--------------------+
| 5.7.25-TiDB-v6.5.0 |
+--------------------+
1 row in set (0.00 sec)

mysql> explain SELECT /*+ INL_JOIN(o,t) */ count(1) AS a FROM test o JOIN ( SELECT SUBSTRING_INDEX(GROUP_CONCAT(id ORDER BY dt DESC), ',', 1) AS id FROM test) t ON t.id = o.id;
+------------------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+------------------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------+
| StreamAgg_12 | 1.00 | root | | funcs:count(1)->Column#9 |
| └─HashJoin_32 | 1.25 | root | | inner join, equal:[eq(Column#11, Column#12)] |
| ├─Projection_18(Build) | 1.00 | root | | cast(substring_index(Column#7, ,, 1), double BINARY)->Column#12 |
| │ └─StreamAgg_22 | 1.00 | root | | funcs:group_concat(Column#16 order by Column#17 desc separator ",")->Column#7 |
| │ └─Projection_34 | 5.00 | root | | cast(test.test.id, var_string(20))->Column#16, test.test.dt |
| │ └─TableReader_29 | 5.00 | root | | data:TableFullScan_28 |
| │ └─TableFullScan_28 | 5.00 | cop[tikv] | table:test | keep order:false, stats:pseudo |
| └─Projection_15(Probe) | 5.00 | root | | cast(test.test.id, double BINARY)->Column#11 |
| └─TableReader_17 | 5.00 | root | | data:TableFullScan_16 |
| └─TableFullScan_16 | 5.00 | cop[tikv] | table:o | keep order:false, stats:pseudo |
+------------------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------+
10 rows in set, 4 warnings (0.01 sec)

mysql> show warnings;
+---------+------+-------------------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------------------------------------------+
| Warning | 1815 | Optimizer Hint /*+ INL_JOIN(o, t) */ or /*+ TIDB_INLJ(o, t) */ is inapplicable |
| Warning | 1105 | Aggregation can not be pushed to tikv because AggFunc `group_concat` is not supported now |
| Warning | 1105 | Aggregation can not be pushed to tikv because AggFunc `group_concat` is not supported now |
| Warning | 1815 | Optimizer Hint /*+ INL_JOIN(o, t) */ or /*+ TIDB_INLJ(o, t) */ is inapplicable |
+---------+------+-------------------------------------------------------------------------------------------+
4 rows in set (0.00 sec)

mysql>
```

**Describe the feature you'd like:**

Correlation column has function processing and supports index join.

**Describe alternatives you've considered:**

N/A

**Teachability, Documentation, Adoption, Migration Strategy:**