Add phpList4 to enable the REST API functionality

[michield]

It will be good to add the new composer based REST API to the container.

[mmx33]

We would like to use the new REST API. Haven't worked much with Phplist, what needs to be done here / how can we help to enable it?

[phpListDockerBot]

Yes, big wish, but no time. If you want to sponsor it, we can try to get it going. There's also this. phpList/phplist3#796

Main issue is that the new API doesn't have a lot of calls yet. If you want a Rest API now, the old one is probably still your best bet https://github.com/phpList/phplist-plugin-restapi/