Add SQL function to show total block numbers in the relation

Hello,

I propose new simple sql query, which shows total block numbers in the
relation.

I now reviewing this patch (https://commitfest.postgresql.org/25/2211/)
and I think,
it is usefull for knowing how many blocks there are in the relation to
determine whether we use VACUUM RESUME or not.

Of cource, we can know this value such as

select (pg_relation_size('t') /
current_setting('block_size')::bigint)::int;

but I think it is a litte bit complex.

Comment and feedback are very welcome.

Regards ,

Yu Kimura