Improve Socialite driver typings + callable typings

[juliangums]

No description provided.

[bert-w]

@juliangums I made some additional changes, since apparently it is also possible for Socialite drivers to return other kinds of response like strings even. The typing of Laravel\Socialite\Two\AbstractProvider::redirect() is in that sense wrong since its docblock says @return \Illuminate\Http\RedirectResponse (which is not always the case).

The other changes in the typings allow the end-user to do some other stuff with the callbacks the plugin provides.

[juliangums]

Looks good. Thanks.