Provide support to optional parameter in OpenID Connect backend

[rgaiacs]

The "OpenID Connect Core 1.0" specification defines a few optional parameters to be used during the authentication request. This optional parameters are

• nonce
• display
• prompt
• max_age
• ui_locales
• id_token_hint
• login_hint
• acr_values

The implemented Open Id Connect backend already implements nonce in

social-core/social_core/backends/open_id_connect.py

Line 133 in 7066da9

params["nonce"] = self.get_and_store_nonce(self.authorization_url(), state)

It will be great to provide users the option to configure other optional parameters by defining some settings, for example for prompt we could have

SOCIAL_AUTH_OIDC_PROMPT = 'select_account'

[nijel]

Partly implemented in #1127