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

[EnricoMi]

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.

arrow/python/pyarrow/includes/libarrow_flight.pxd

Lines 133 to 139 in 093655c

	cdef cppclass CFlightEndpoint" arrow::flight::FlightEndpoint":
	CFlightEndpoint()
	CTicket ticket
	vector[CLocation] locations
	optional[time_point] expiration_time
	c_string app_metadata

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: [FlightRPC][C++] Precision of arrow.flight.Timestamp is OS-dependent #44679

[EnricoMi]

Splitting this PR into the C++ part (the first commit) and the Python part (the second commit) requires the Python workaround that is being removed in the second commit to be adjusted in the first commit: #44680 (comment)

To avoid throw-away work, I'd prefer doing this in one PR.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit d534e77.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details.