RFC: Add plugin system to allow extension of base system

[joniw]

Hello dear development team,

thank for this awesome project and the time you are spending on it. I am opening this issue as a followup to my issue #2412.

I totally understand that you want to focus on the main goals of the project. Every code change will generate questions from users and add additional support effort. On the other hand and every administrator and user has different needs or ideas, what the "perfect" mail system should look like.

In order to meet the requirement of keeping the base small but also allow cusomization by administrators, i would like to discuss the possibility to add a plugin system to mailu.

My current imagination is to allow some kind of hookups in the current ui with content blocks extended by new routes which implement the actual functionality and these plugins should be manageable in the ui.

What are your thoughts on this proposal?

With regards

Joniw