[2.x] Resolve socialite user action

[joelbutcher]

Some develops may want to make changes to the way this package resolves users. For example, they may want to call the ->stateless() method on socialite. This PR fixes this by deferring the resolution of the socialite user to a default contract implementation.

Developers can now override how Socialite users are resolved by implementing the ResolvesSocialiteUsers contract and specifying the action they wish to use in their SocialstreamServiceProvider:

public function boot()
{
    \JoelButcher\Socialstream\Socialstream::resolvesSocialiteUsersUsing(ResolveSocialiteUser::class);
}