Allow configurable permissions to be set when creating group memberships.

### Is your feature request related to a problem? Please describe.

Want to set configurable_permissions at the same time when creating group memberships.

### Describe the solution you'd like

I think a good way to do this is to add an addMembership method to the BoxGroup class that corresponds to Configurable Permission.

We believe that it is possible to set the Permission when an administrator is added by performing the same process as the configurable permission implemented in the Box Python SDK.

https://github.com/box/box-python-sdk/blob/master/docs/usage/group.md#add-user-to-group

### Describe alternatives you've considered

N/A

### Additional context

N/A

Comienza con la clase BoxGroup y sus métodos existentes para crear membresías de grupo; después, compara sus parámetros de solicitud con la documentación enlazada del Box Python SDK sobre permisos configurables. Se considera completado cuando el Java SDK pueda establecer configurable_permissions al crear una membresía, con un comportamiento coherente con el flujo documentado del Python SDK.

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