Allow plugins to implement authentication backends

[HimbeerserverDE]

This needs further consideration. Important implementation questions:

• New behavior of the config option?
• How to handle conflicts, i.e. multiple plugins trying to set the auth backend?
• Move builtin backends to their own plugins? Probably not as it has the potential to cause a lot of confusion and inconvenience.
• Expose builtin backends (to make wrapping them possible)? Yes

This makes it possible to build custom, more complex authentication systems for purposes like sharing them with a website or to implement a custom storage method.