Allow to set custom object identifier builder

[ljacqu]

After #48

[ebuildy]

Hello, just wondering if with this issue achieved, this will be possible to access/register service with a String instead of Class? thanks.

[ljacqu]

That‘s a great question. Essentially no, I just realize, since the methods on the Injector interface all use Class parameters. What‘s your use case and/or how would you ideally want to use Strings for classes? Just so we have some context :)