Implement LinkedIn via OpenID-Connect

[steinkel]

Setup:

• Create an application in linkedIn https://developer.linkedin.com/
• Ensure the application is approved
• Go to your application dashboard, under "Products" tab and enable "Sign In with LinkedIn using OpenID Connect" https://i.imgur.com/xL7nnqT.png
• Configure your users.php with the following provider:

    'OAuth' => [
        'providers' => [
            'linkedInOpenIDConnect' => [
                'service' => \CakeDC\Auth\Social\Service\OpenIDConnectService::class,
                'className' => \League\OAuth2\Client\Provider\LinkedIn::class,
                'mapper' => \CakeDC\Auth\Social\Mapper\LinkedInOpenIDConnect::class,
                'options' => [
                    'redirectUri' => Router::fullBaseUrl() . '/auth/linkedInOpenIDConnect',
                    'linkSocialUri' => Router::fullBaseUrl() . '/link-social/linkedInOpenIDConnect',
                    'callbackLinkSocialUri' => Router::fullBaseUrl() . '/callback-link-social/linkedInOpenIDConnect',
                    'defaultScopes' => ['email', 'openid', 'profile'],
                    // you can check your credentials in the "Auth" tab of your application dashboard in LinkedIn developers page
                    'clientId' => 'YOUR_CLIENT_ID',
                    'clientSecret' => 'YOUR_CLIENT_SECRET',
                ],
            ],
        ],
    ],

[steinkel]

Note: when merged, we'll need to update the documentation in cakedc/users to reflect this new social provider. A migration guide from the old OAuth2 provider could be a good idea too...

Note2: unit tests being finished now

[codecov-commenter]

Codecov Report

Merging #81 (7d73629) into 7.next-cake4 (9e126f8) will decrease coverage by 1.45%.
The diff coverage is 47.61%.

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@@                Coverage Diff                 @@
##             7.next-cake4      #81      +/-   ##
==================================================
- Coverage           90.32%   88.87%   -1.45%     
- Complexity            318      330      +12     
==================================================
  Files                  47       49       +2     
  Lines                 775      944     +169     
==================================================
+ Hits                  700      839     +139     
- Misses                 75      105      +30     

Files	Coverage Δ
src/Social/Mapper/LinkedInOpenIDConnect.php	100.00% <100.00%> (ø)
src/Social/Service/OpenIDConnectService.php	45.00% <45.00%> (ø)

... and 19 files with indirect coverage changes

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