Add facet

Hello,

How do you format the data to specify facets (=edge attributes) ? I would like to do something like (A)-distance=1->(B).

```python
txn = client.txn()
data = ?
try:
  assigned = txn.mutate(set_obj=data)
  txn.commit()
finally:
  txn.discard()
```

Thanks a lot,
lapwat