Adding authentication via JSON REST API

[msfuture]

Changes proposed in this pull request:

• adding REST JSON API for external user authentication:
• checks REST endpoint and gets optional and additional user data like groups and displayname

[violoncelloCH]

thanks for your contribution @msfuture
I'll take a closer look at it as soon as I find the time to do so...

[msfuture]

By the way: I'm already testing this type of authentication with about 400 accounts involved. No errors and automatical group assignment made life so much easier for my colleagues.

[kesselb]

There is a wrapper for Guzzle and Curl: IClient (and IClientService as factory`). An advantage is that a proxy configuration (at config.php) is automatically used.

[violoncelloCH]

thanks for the hint @kesselb !
so that should definitely be used (also by the other backends)... @msfuture could you take a look at this?
https://github.com/nextcloud/server/blob/master/lib/private/Http/Client/Client.php

[msfuture]

I'll take a look at this!

[msfuture]

Time went by so fast... But I finally got to it.