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

从 BoxGroup 类及其现有的组成员关系创建方法入手，然后将它们的请求参数与链接的 Box Python SDK 关于可配置权限的文档进行比较。完成标准是 Java SDK 能够在创建成员关系时设置 configurable_permissions，并且行为与 Python SDK 文档所述的流程一致。

由索引模型根据 Issue 内容生成。