feat: add bidirectional edges to network motif finding

The only type of edge atm that is valid is `(a)-[e1]->(b)` and I would like to add support for `(a)<-[e1]->(b)` in [GraphFrame.find()](https://graphframes.github.io/graphframes/docs/_site/api/python/graphframes.html#graphframes.GraphFrame.find). This can include support for backwards facing edges: `(a)<-[e1]-(b)`.