[C++] [Python] Add support for exporting extension types

[asfimport]

It would be nice to be able to export extension fields and arrays, i.e.

import pyarrow

class UuidType(pyarrow.PyExtensionType):
    def __init__(self):
        super().__init__(pyarrow.binary(16))

    def __reduce__(self):
        return UuidType, ()

field = pyarrow.field("aa", UuidType())
field._export_to_c(pointer)

would not raise

pyarrow.lib.ArrowNotImplementedError: Exporting extension<arrow.py_extension_type<UuidType>> array not supported

Reporter: Jorge Leitão / @jorgecarleitao
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #11071

_{Note: This issue was originally created as ARROW-13855. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
Issue resolved by pull request 11071
#11071