This commit adds the handling of graph *properties*, i.e., data attached
to nodes or edges (commit timestamps, commit messages, content lengths,
...) to swh-graph.
The class WriteNodeProperties is used to extract the node properties
from the ORCGraphDataset and write them in separate files, in compressed
format. The properties can then be read using the SwhGraphProperties
class.
The compression pipeline and the tests were all changed to use the new
dataset format.
Unfortunately there are a lot of interlocking parts and refactors that I had to
work on in parallel, so this commit is not as... atomic as it could be.
The CI also won't pass until a new version of WebGraph is released.