Add method to get encryption keys

As `MutableData` basically mirrors [`MDataInfo`](https://docs.rs/safe_app/0.9.0/safe_app/struct.MDataInfo.html), could there be a method added to retrieve the encryption keys?

Perhaps not as a method, but as properties to the instance. Might as well offer `name` and `type_tag` as properties too. Also, `getNameAndTag` is asynchronous, but I see no reason for it to be.