Add support for per-app config

[sabondano]

Why:

• Now that Ueberauth version 0.6+ supports per-app configuration, we'd like to update uberauth_auth0 strategy to support per-app configuration also.

  We'd like to config our apps like this, in our configuration file (config/config.exs):

  # Configures Ueberauth
  config :my_app, Ueberauth,
  providers: [
    auth0: {Ueberauth.Strategy.Auth0, [otp_app: :my_app]}
  ]

  and to use Ueberauth from a controller like this:

  plug Ueberauth, otp_app: :my_app

• Issue link: n/a

This change addresses the need by:

• Editing Ueberauth.Strategy.Auth0.handle_request!/1 and Ueberauth.Strategy.Auth0.handle_callback!/1 to pass along the configured :otp_app, found in the conn, to a function that needs it in the body of these two functions.
• Editing Ueberauth.Strategy.Auth0.OAuth to get config from otp_app when an otp_app is provided to authorize_url!/2 and get_token!/2.

[sntran]

Thank you for contributing!

I'll have to take a look later this week if you don't mind.

[sabondano]

@sntran My pleasure 😊. No problem and thank you!

[sabondano]

Thank you for contributing!

I'll have to take a look later this week if you don't mind.

👋 @sntran did you have a chance to look at this last week 😅?

[sntran]

Thanks again @sabondano !