Define DatumGetInt8 function.

Hi hackers!

I am currently involved in the Cloudberry kernel rebase process[0]. We
are rebasing [0] which is based on pg-14 on pg-16 kernel. During this
process I noticed rebase conflicts introduced  by c8b2ef0. This commit
defines a number of include functions for datum conversion.

During this rebase resolution, I noticed that there is an Int8GetDatum
function, but there is no DatumGetInt8, which I want to use. All other
inline functions seem to be provided in pairs by postgres.h. So it
looks convenient to define DatumGetInt8 in postgres.h?

FPA doing just that.


[0] https://github.com/apache/cloudberry

-- 
Best regards,
Kirill Reshke