Support UTF-8 data in `JsonProperty`

We have datastore entities with a property that holds JSON data. The entities itself were written using a datastore library in a different language than Python. The data includes unescaped UTF-8 characters (which should be legal in JSON, and is generally well supported).

When using an NDB `JsonProperty` to access this data from within Python, however, this breaks. This is because `JsonProperty` only supports ascii data:

```python
def _from_base_type(self, value):
if not isinstance(value, str):
value = value.decode("ascii")
return json.loads(value)
```

Could `JsonProperty` be adapted to decode `utf-8`, to have better interoperability between different languages working on the same data?