Limit OpenID Connect logins to users of specific user backend

[DeepDiver1975]

Description

This adds a config option to limit users which are logged in via OpenID Connect to specific user backends - e.g. LDAP

Config.php:

<?php
$CONFIG = [
	'openid-connect' => [
		'allowed-user-backends' => ['LDAP']

Motivation and Context

In more security aware environments this can be of interest ..... 🙈

How Has This Been Tested?

• ✋

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:

Open tasks:

• ...

[codecov]

Codecov Report

Merging #100 into master will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #100      +/-   ##
============================================
+ Coverage     93.65%   93.81%   +0.15%     
- Complexity      114      117       +3     
============================================
  Files            10       10              
  Lines           394      404      +10     
============================================
+ Hits            369      379      +10     
  Misses           25       25              

Impacted Files	Coverage Δ	Complexity Δ
lib/Service/UserLookupService.php	100.00% <100.00%> (ø)	12.00 <3.00> (+3.00)

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[jnweiger]

The release is back in dev due to this.

[mmattel]

This is docs relevant because it contains (as far I can see) a new config option !
Please file a docs issue so it can be tracked. In case of merging a PR.

[DeepDiver1975]

@mrkwenzel #101 - issue opened ....

[DeepDiver1975]

@mmattel owncloud/docs#2783