Implement NetworkStatusDiff Document

[jbrown299]

Current version Stem library doesn't have network-status-diff document.

https://github.com/torproject/torspec/blob/e576760a2136f0efb917bf7fb47f4fe8a22cd7d9/proposals/140-consensus-diffs.txt

You can try take some code from torpy https://github.com/torpyorg/torpy/blob/master/torpy/documents/network_status_diff.py#L92

[atagar]

Interesting idea! Do we have any concrete use cases for this? It would be a pity to implement this if we don't have a user for it.