Add operation_member function

This function is intended to index hive.operation members in similar
fashion to jsonb operators `->` and `->>`.

E.g.
```sql
Select _operation_member(body, 'value.author') from operations as op join operation_types as t on (op.op_type_id=t.id) where t.name = 'hive::protocol::comment_operation';
```
Will return author of comment operations.