pg_plan_advice: fix parsing underscore in numbers

Hi,

I’m playing with pg_plan_advice and found a small issue.

PG supports underscores between digits; for example, 1_0 is parsed as 10:
```
evantest=# SELECT 1_0;
?column?
----------
10
(1 row)
```

pg_plan_advice seems to intend to support the same syntax, based on pgpa_scanner.l:
```
decdigit [0-9]
decinteger {decdigit}(_?{decdigit})*
```

But it then uses strtoint() to parse the token, so an underscore causes a parse error:
```
evantest=# SET pg_plan_advice.advice = 'SEQ_SCAN(x#1_0)';
ERROR: invalid value for parameter "pg_plan_advice.advice": "SEQ_SCAN(x#1_0)"
DETAIL: Could not parse advice: integer out of range at or near "1_0"
```

The attached patch fixes this by using pg_strtoint32_safe() in the same way as the core scanner. With the fix, 1_0 works:
```
evantest=# SET pg_plan_advice.advice = 'SEQ_SCAN(x#1_0)';
SET
```

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/