Added Keycloak as a new Auth Client

[eluhr]

Q	A
Is bugfix?	no
New feature?	yes
Breaks BC?	no
Tests pass?	yes
Fixed issues	-

With this pull request I have added a new Auth Client for Keycloak. Here is an example of how it can be configured:

// ... 
'components' => [
    // ...
    'authClientCollection' => [
        'class' => 'yii\authclient\Collection',
        'clients' => [
            'facebook' => [
                'class' => 'Da\User\AuthClient\Keycloak',
                'clientId' => 'keycloak_client_id',
                'clientSecret' => 'keycloak_client_secret',
                'issuerUrl' => 'http://keycloak/realms/your-realm'
            ]
        ]
    ]
]

[maxxer]

If you don't mind, I'd add the configuration example in the class comments itself, to avoid having it lost in this PR

[eluhr]

I've just added it