arrow

GH-44679: [C++][Python] Fix Flight Timestamp precision, revert workaround from #43537 (#44681)

### What changes are included in this PR?
Make `arrow.flight.Timestamp` nanoseconds precision and OS-independent.

Use the `CTimePoint` for `FlightEndpoint.expiration_time` to have OS-independent nanoseconds precision.

https://github.com/apache/arrow/blob/093655c60783321c786a8e69632f185d37520f4d/python/pyarrow/includes/libarrow_flight.pxd#L133-L139

This reverts workaround for `expiration_time` from  #43537.

### Are these changes tested?
Existing unit tests are adjusted to the new precision.

### Are there any user-facing changes?

**This PR includes breaking changes to public APIs.**

The values of `FlightEndpoint.expiration_time` now have nanoseconds precision on any operating system.

* GitHub Issue: #44679

Authored-by: Enrico Minack <github@enrico.minack.dev>
Signed-off-by: David Li <li.davidm96@gmail.com>