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

Start with the BoxGroup class and its existing group-membership creation methods, then compare their request parameters with the linked Box Python SDK documentation for configurable permissions. Done means the Java SDK can set configurable_permissions while creating a membership, with behavior consistent with the documented Python SDK flow.

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