[1.x] Add Jetstream route prefix config

[MaFx]

This PR adds config for prefixing jetstream routes in the same pattern that is used in Laravel Fortify route binding.

This allows adding prefix like /admin to routes if the application requires changes in url structure.

[taylorotwell]

Why? Is there a particular reasons many applications would want to customize this?

[MaFx]

An example of the implementation would be using the /admin prefix for all authenticated routes.
It seems that the only other alternative for changing route paths is deregistering the routes from Jetstream and re-binding all of them in app level through Laravel\Jetstream\Jetstream::$registersRoutes.

This exact implementation already exists in Fortify and the consistency of being able to do the same in Jetstream would make sense https://github.com/laravel/fortify/blob/1.x/src/FortifyServiceProvider.php#L128

[bilogic]

@MaFx I thought it would be good if this option also shows up in the config/jetstream.php.