Extend _type_map with Array

I have some tables containing Array columns in a database. When `DremioDialect.get_columns()` is called `ctype = _type_map[col[1]]` throws an `KEYERROR 'ARRAY'`.  
This can be solved with extending `_type_map` in `base.py` and `query.py` with:   
```
_type_map = {
     ...
    'ARRAY': types.ARRAY,
    'array': types.ARRAY,
}
```

