Add table access method as an option to pgbench

Hi Hackers,

I noticed that there is a table access method API added starting from
PG12. In other words, Postgresql open the door for developers to add
their own access methods, for example, zheap, zedstore etc. However, the
current pgbench doesn't have an option to allow user to specify which
table access method to use during the initialization phase. If we can
have an option to help user address this issue, it would be great. For
example, if user want to do a performance benchmark to compare "zheap"
with "heap", then the commands can be something like below:

pgbench -d -i postgres --table-am=heap

pgbench -d -i postgres --table-am=zheap

I know there is a parameter like below available in postgresql.conf:

#default_table_access_method = 'heap'

But, providing another option for the end user may not be a bad idea,
and it might make the tests easier at some points.

The attached file is quick patch for this.

Thoughts?

Thank you,

--
David

Software Engineer
Highgo Software Inc. (Canada)
www.highgo.ca