feat: implement inplace parameter for DataFrame.drop

[tswast]

This commit implements the inplace parameter for the DataFrame.drop method.

When inplace=True, the DataFrame is modified in place and the method returns None. When inplace=False (the default), a new DataFrame is returned.

Unit tests have been added to verify the functionality for both column and row dropping.

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Fixes b/385199126 🦕

[tswast]

@jialuoo please take a look. I assigned to you because Shuowei is OOO.

In [1]: import random

In [2]: random.choice(['chelsealin', 'garrettwu', 'jialuo', 'sycai', 'tbergeron'])
Out[2]: 'jialuo'