BUG #18568: BUG: Result wrong when do group by on partition table!

The following bug has been logged on the website:

Bug reference: 18568
Logged by: Webbo Han
Email address: 1105066510(at)qq(dot)com
PostgreSQL version: 16.3
Operating system: centos 7.6
Description:

First, we create one case-insensitive collation use ICU:
```sql
CREATE COLLATION case_insensitive (
provider = icu,
locale = 'und-u-ks-level2',
deterministic = false
);
```

Then, we create the partition table, meanwhile we set the collation of
column c to `case_insensitive`,
and set partkey's collation to 'C'.
```sql
SET enable_partitionwise_aggregate TO true;
SET enable_partitionwise_join TO true;
SET max_parallel_workers_per_gather TO 0;
SET enable_incremental_sort TO off;
CREATE TABLE pagg_tab (c text collate case_insensitive) PARTITION BY LIST(c
collate "C");
CREATE TABLE pagg_tab_p1 PARTITION OF pagg_tab FOR VALUES IN ('a', 'b',
'c', 'd');
CREATE TABLE pagg_tab_p2 PARTITION OF pagg_tab FOR VALUES IN ('e', 'f',
'A');
CREATE TABLE pagg_tab_p3 PARTITION OF pagg_tab FOR VALUES IN ('B', 'C',
'D', 'E');
INSERT INTO pagg_tab SELECT substr('abcdeABCDE', (i % 10) +1 , 1) FROM
generate_series(0, 2999) i;
ANALYZE pagg_tab;
```

We do group by on the table pagg_tab use `case_insensitive` collation, we
hope group key is case-insensitive.
but we find the execution result is not what we expected.
```shell
postgres=# SELECT c collate case_insensitive, count(c) FROM pagg_tab GROUP
BY c collate case_insensitive;
c | count
---+-------
A | 300
e | 300
E | 300
D | 300
C | 300
B | 300
d | 300
c | 300
b | 300
a | 300
(10 rows)
```

The reason is the function group_by_has_partkey() do not check partkey's
collation, that lead to explain error.
```shell
postgres=# EXPLAIN SELECT c collate case_insensitive, count(c) FROM
pagg_tab GROUP BY c collate case_insensitive ;
QUERY PLAN
--------------------------------------------------------------------------------------
Append (cost=12.00..60.15 rows=10 width=10)
-> HashAggregate (cost=12.00..12.02 rows=2 width=10)
Group Key: pagg_tab.c
-> Seq Scan on pagg_tab_p2 pagg_tab (cost=0.00..9.00 rows=600
width=2)
-> HashAggregate (cost=24.00..24.04 rows=4 width=10)
Group Key: pagg_tab_1.c
-> Seq Scan on pagg_tab_p3 pagg_tab_1 (cost=0.00..18.00
rows=1200 width=2)
-> HashAggregate (cost=24.00..24.04 rows=4 width=10)
Group Key: pagg_tab_2.c
-> Seq Scan on pagg_tab_p1 pagg_tab_2 (cost=0.00..18.00
rows=1200 width=2)
(10 rows)
```

So, group_by_has_partkey() need to verify if the partkey's collation matches
the groupkey,
meanwhile, if groupkey is RelabelType node and it's collation equal to
partkey's, it should
also set variable `found` to true.