pg_amcheck option to install extension

Hi,

Peter Geoghegan suggested that I have the cross version upgrade checker
run pg_amcheck on the upgraded module. This seemed to me like a good
idea, so I tried it, only to find that it refuses to run unless the
amcheck extension is installed. That's fair enough, but it also seems to
me like we should have an option to have pg_amcheck install the
extension is it's not present, by running something like 'create
extension if not exists amcheck'. Maybe in such a case there could also
be an option to drop the extension when pg_amcheck's work is done - I
haven't thought through all the implications.

Given pg_amcheck is a new piece of work I'm not sure if we can sneak
this in under the wire for release 14. I will certainly undertake to
review anything expeditiously. I can work around this issue in the
buildfarm, but it seems like something other users are likely to want.

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com