Add extension pg_ivm

[maximepvrt]

Can you add https://www.postgresql.org/about/news/pg_ivm-12-released-2489/ https://github.com/sraoss/pg_ivm

[tbussmann]

While we cannot include every extenion available, we need to find a messure on what is demanded often, so thanks for raising your voice.

Even if an extention is not included in the distrubution of Postgres.app, most extensions can be built and added by the users themself. We wanted to document these steps for quite some time and I planned to provide a list of tested working extensions. If you search the issues, you can find some recipies I gave in the past. Generally, it boils down to the steps I just mentioned in #683, which work for pg_ivm as well.

Annother option that should work and simplyfies the process is to install the pgxnclient