[C++][Python] Support ExtensionType arrays in more kernels

From a quick test (through Python), it seems that `slice` and `take` work, but the following not:

- `cast`: it could rely on the casting rules for the storage type. Or do we want that you explicitly have to take the storage array before casting?
- `dictionary_encode` / `unique`

**Reporter**: [Joris Van den Bossche](https://issues.apache.org/jira/browse/ARROW-5890) / @jorisvandenbossche
**Watchers**: [Rok Mihevc](https://issues.apache.org/jira/browse/ARROW-5890) / @rok
#### Related issues:
- [[C++] C++ array kernels framework and execution buildout (umbrella issue)](https://github.com/apache/arrow/issues/25028) (is a child of)
- [[C++][CSV] Add support for Extention type in csv reader](https://github.com/apache/arrow/issues/23392) (relates to)
- [[C++][Python] Support casting an Extension type to its storage type](https://github.com/apache/arrow/issues/24086) (relates to)

_{**Note**: *This issue was originally created as [ARROW-5890](https://issues.apache.org/jira/browse/ARROW-5890). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}