title: Symfony 3 authentication provider fallback
body: <p>I have implemented authentication mechanism on some mobile application using <a href="https://github.com/lexik/LexikJWTAuthenticationBundle" rel="nofollow">JWTBundle</a> with symfony 3. Until now the process requires users to submit both their email and  password in order to authenticate. This works great.</p>
  
  <p>Today I would like to grant access/create_account using Facebook authentication.</p>
  
  <p>From the mobile app, users will access the application without submitting any password but instead FB will probably return user's Facebook identifier along with some other info. I will then post those datas to <em>login_check</em> route.</p>
  
  <p>At this point <strong>I need a way to check (at the very beginning of request processing flow) whether login_check POST datas are standard username/pwd credentials (which I guess are handled through daoauthenticationprovider by default ? which in turn pass the processing to JWT in order to create a authentication token) and if not, fallback to another custom XXAuthenticationProvider to handle those datas (eg. if a facebook identifier is present then lookup the user account with FB API, do stuff … then create a JWT token).</strong></p>
  
  <p>I read couples of articles dealing with Symfony's Security components but none explained the whole thing clearly neither exposed a way to proceed. I still have difficulties to figure out how I could hook into the security firewall to achieve this.</p>
  
  <p>Is this a way to go and how can I achieve this ?<br>
  Thank you.</p>