[FEA] Define `__hash__` on CategoricalDtype

**Is your feature request related to a problem? Please describe.**
Would be nice if `hash(CategoricalDtype(...))` was supported

```
In [9]: import cudf

In [10]: import pandas

In [11]: hash(cudf.CategoricalDtype(list("abc")))
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Cell In[11], line 1
----> 1 hash(cudf.CategoricalDtype(list("abc")))

TypeError: unhashable type: 'CategoricalDtype'

In [12]: hash(pandas.CategoricalDtype(list("abc")))
Out[12]: 1532899084736511412
```

**Describe the solution you'd like**
Define `__hash__` on `CategoricalDtype`

**Describe alternatives you've considered**
Custom hash based on order and categories

**Additional context**
Add any other context, code examples, or references to existing implementations about the feature request here.