Implement truncate for DataFrame and Series

Add an implementation of truncate for DeferredDataFrame, and DeferredSeries. It should be fully unit tested with some combination of pandas_doctests_test.py and frames_test.py.

https://github.com/apache/beam/pull/14274 is an example of a typical PR that adds new operations. See https://lists.apache.org/thread.html/r8ffe96d756054610dfdb4e849ffc6a741e826d15ba7e5bdeee1b4266%40%3Cdev.beam.apache.org%3E for background on the DataFrame API.