Support networks that POST to the authorization callback

Problem/Motivation

Some providers, such as "Sign in with Apple", perform a POST request to the authorization callback / redirect URI and provide the state parameter in the POST body.

However, Social Auth's OAuth2ControllerBase::processCallback() currently assumes the state parameter is provided as a query argument, resulting in authorization failing and Social Auth outputting the message Login failed. Invalid OAuth2 state..

Proposed resolution

Instead of checking for state in Symfony\Component\HttpFoundation\Request::query (which provides only the query parameters), use Symfony\Component\HttpFoundation\Request::get (which checks both query parameters and POST body parameters).

Remaining tasks

Review attached patch.

User interface changes

n/a

API changes

n/a

Data model changes

n/a