Set/retrieve ordered flag for DictEncoded

The Arrow dictionary format allows for an `ordered` flag indicating that the order of the elements in the dictionary should be preserved. This corresponds to the `ordered` property of a `CategoricalArray` in Julia. It would help to preserve the ordered property when writing the `DictEncoded` value and also to preserve it when reading. A conversion method to `CategoricalArray` could then set the ordered flag.

As an example of the current behavior, in Julia if I write an ordered CategoricalArray
```julia
julia> using Arrow, CategoricalArrays

julia> levs = ["d", "c", "b", "a"];

julia> a = CategoricalArray(repeat(levs, inner=3); levels = levs, ordered=true);

julia> b = CategoricalArray(repeat(levs, inner=3); levels = levs, ordered=false);

julia> Arrow.write("/tmp/test.arrow", (; a = a, b = b))
"/tmp/test.arrow"
```
both end up in python as unordered.
```python
Python 3.9.7 | packaged by conda-forge | (default, Sep 29 2021, 19:20:46)
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pyarrow.feather as fthr
>>> fthr.read_table('/tmp/test.arrow')
pyarrow.Table
a: dictionary not null
b: dictionary not null
>>>
```

Start by tracing how DictEncoded values are written and read, then inspect the conversion to CategoricalArray. Use the Julia example and pyarrow output in the issue to verify that ordered=true and ordered=false survive both serialization and conversion.

Written by the indexing model from the issue text.