Hello,

This is my first contribution to GNU Guix, and I hope this patch is in the correct format.

Attached is a small patch to gnu/services/databases.scm that adds an option to explicitly
specify the UID / GID for the postgres user, as well as an option not to create the service
account at all.

I added some documentation as well and provided a detailed reasoning and background
in the commit message.

Hope this is useful and looking forward to hearing back to you,

Martin