Added pg_cron to Alpine Linux

I've added pg_cron to the Alpine Linux distribution; this because alpine is heavily used within docker environments, so pg_cron can be used within a docker container which runs postgresql on Alpine.

Currently it's only available for the latest 'edge' branch under testing.
https://pkgs.alpinelinux.org/packages?name=pg_cron&branch=&repo=&arch=&maintainer=

Architectures:
- aarch64
- armhf
- x86
- x86_64

Question:
Currently the latest version is a release candidate, is there any information when the final 1.0.0 will be released ?