[C++][Python] Support casting an Extension type to its storage type

[asfimport]

Currently, casting an extension type will always fail: "No cast implemented from extension<arrow.py_extension_type> to ...".

However, for casting, we could fall back to the storage array's casting rules?

Reporter: Joris Van den Bossche / @jorisvandenbossche
Assignee: Krisztian Szucs / @kszucs

Related issues:

• [Python] Failing test with pandas master for extension type conversion (blocks)
• [C++] Support casting from storage type to extension type (relates to)
• [C++][Python] Support ExtensionType arrays in more kernels (is related to)

PRs and other links:

• GitHub Pull Request #6633

_{Note: This issue was originally created as ARROW-7858. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 6633
#6633