[PATCH] pg_get_domain_ddl: DDL reconstruction function for CREATE DOMAIN statement

Hello everyone, 

I have tested the v4 patch and observed that the following scenario requires support. Accordingly, I have prepared
versionv5: v5-0001 is fully consistent with the v4 patch, and v5-0002 restricts input to domains exclusively, with an
additionalsimple test case attached.
 

```sql
postgres=# create table test_table(a int);
CREATE TABLE
postgres=# SELECT pg_get_domain_ddl('test_table');
ERROR:  cache lookup failed for type 0
```

-- 
Regrads,
Man Zeng