Added Oauth2ClientInterface for service decorating

[sadikoff]

Example:

namespace App\Client;

class CacheableClient implements OAuth2ClientInterface
{
    private $client;
    private $cache;

    public function __construct(OAuth2ClientInterface $client, Cache $cache)
    {
        // ...
    }

    // override all public functions, call the method on the internal $this->client object
    // but add caching wherever you need it
}

services:
    app.azure_client:
        class: App\Client\CacheableClient
        decorates: knpu.oauth2.client.azure

closes #134

[FlintCIBot]

Lint errors were found. A patch is also available.

Please see the report: https://flintci.io/repositories/4653/analyses/12046

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[weaverryan]

@sadikoff This looks perfect! Could you also add a small section in the README file, maybe call it "Extending/Decorating Client Classes"?

[sadikoff]

@weaverryan added, but it needs your review.

[weaverryan]

Nice work!

[aeimer]

Pretty cool! Thanks :)