Make `URLSession` Configurable

### Is your feature request related to a problem? Please describe.
We are unable to configure the `URLSession` beyond the default settings, see: https://github.com/box/box-ios-sdk/blob/a395222866dd441f474f25fbe15837ae37662e68/Sources/Network/BoxNetworkAgent.swift#L61

This restricts our ability to create background `URLSession`s with custom configurations, particularly concerning setting session identifiers and shared container identifiers.

### Describe the solution you'd like
We propose updating the `BoxNetworkAgent` to create the `URLSession` based on some sort of configuration. One way could be to extend the `BoxSDKConfiguration` to accept a `URLSessionConfiguration`.

### Describe alternatives you've considered
There isn't really an alternative. Since we're implementing a File Provider extension, we require the `URLSession`s to be configured as a background session.

### Additional context
We are willing to contribute to implementing this feature and are open to creating a pull request. However, we would appreciate guidance from you to ensure the feature aligns with your best practices for your library.