Re:Add pg_get_publication_ddl function

Hi,

I haven't tested your patch yet, but I noticed a few points that may need adjustment:

Based on your code logic, the pub parameter passed here can never be NULL — if the corresponding PUBLICATION does not
exist,an error should have already been thrown earlier in the code flow. 
 
Therefore, the following code block can be removed, and the usage of return (Datum) NULL; in this block is also
incorrect:
```
    if (pub == NULL)
        return (Datum) NULL;
```
pubtuple is not being freed — please use heap_freetuple to release it.

The format string "%sALL SEQUENCE" should be corrected to "%sALL SEQUENCES".

--
Regards,
Man Zeng
www.openhalo.org