[2.x] Make the guard configurable

[bambamboole]

I am building a Laravel package which uses sanctum. It was really a missing peace in the eco system. Thanks for this!

However, since I have an additional guard in my package, I had problems to make the described stateful requests.

There was also no way to swap out the implementation via the container because it is instantiated with new instead of using the container.

This little change would give me the possibility to go on with this package. I think there is also a discussion about this topic in #20

There are no breaking changes.

[iSWORD]

This is very helpful and needs to be added to the config file.