Added support for ns #1169

[zaryab-ali]

@kevinjqliu made the changes suggested in the previous comments and added the test case for the functionality

[zaryab-ali]

@kevinjqliu i have a few questions about the these test case scenarios

• the first scenario is already present i think, test_pyarrow_time64_us_to_iceberg function checks with us precision
• secondly, can you please explain a bit about the test case you want me to write on the read/write path, what file should that be in and what specifically should it test for

[JE-Chen]

You can find some information in my previous PR. https://github.com/apache/iceberg-python/pull/1206

[kevinjqliu]

• [x] Time64Type with us precision. This was already supported by the previous code.
• [ ] Time64Type with ns precision, _downcast_ns_timestamp_to_us not set. Should error
• [ ] Time64Type with ns precision, _downcast_ns_timestamp_to_us is set. Should downcast precision to us
• [ ] Able to write pyarrow dataframe with a column with Time64Type with ns precision.

You can use #848 as an example where timestamp downcast was implemented