I found that the package for this plugin exists (or at least did when I installed it a couple months ago) on 18.04 if you setup the repo for MariaDB 10.2, but that it doesn't exist in the repo for 10.3. I did a bunch of digging and couldn't find out why, and when I sent an email to the list about it, nobody ever replied.
For now, unless you decide to compile it yourself, you might want to try running MariaDB 10.2. you should then be able to use this plugin.
If anyone else could also shed more light on this, that would be awesome :) I'd still prefer to go with my original choice of 10.3.