Add ability to override access token request headers

[JasonTolliver]

This moves the currently hardcoded getAccessTokenResponse into their own method. This allows children providers to construct their own headers as needed.

A perfect example of the necessity of this feature is the newer Pinterest v5 API. In order to exchange the code for an access token, they require an Authorization header composed of a base64 encoded client_id and client_secret. Without this change, the Pinterest Provider would need to entirely reimplement the getAccessTokenResponse method. With this change, the Pinterest Provider could simply:

protected function getTokenHeaders()
{
    return array_merge(
        parent::getTokenHeaders(),
        [
            'Authorization' => 'Basic ' . base64_encode("{$this->clientId}:{$this->clientSecret}"),
        ]
    );
}