Plugin architecture to support user add-ons

[billz]

Discussed in #1052

^{Originally posted by billz November 13, 2021}
One idea I've played with is adding plugin support for RaspAP. This would let other developers build enhancements for the project without modifying the core code. Examples include this interesting fork of RaspAP that adds RPiPlay integration. While this may not appeal to everyone, there are probably some users who would add this to RaspAP if it could be done simply.