External user onboarding / linking: LDAP

**is your feature request related to a problem? Please describe.**
Having a simple config to manage your users is nice for a simple small personal setup, but as soon as you start to collaborate with different users or maybe also have other self hosted services this will become lots of duplication for onboarding users.

**Describe the idea / solution you'd like**
It would be nice to have the definitions of users have the possibility to come from somewhere else.
Primarily the passwords. if we introduce groups (#188) this can also help to assign permissions via LDAP to different shares easily.
One of the simplest, most flexible solutions would probably be LDAP support, which you can use to query users / groups.

**Additional considerations**
There also exists forward http headers that can be used for auth. I think this is already possible with the IDP SSO?
One issue i can see with this forward auth is that I dont know how it will play with other services like SFTP, i dont think they support something like this - you could think about giving the user the option to set dedicated passwords after login for this, but then you will have to manage service specific passwords again, but we wanted to delegate auth to the proxy so we didnt win anything...

Most flexible would probably be supporting LDAP too, this way you could also think about supporting SSH keys for the SFTP service