[FEA] Support the case= argument to str.contains(...)

[shwina]

In Pandas .str.contains() function accepts a case= argument that ignores case when searching for substrings:

>>> s = pd.Series(['a', 'A', 'b'])
>>> s.str.contains('a')
0     True
1    False
2    False
dtype: bool
>>> s.str.contains('a', case=False)
0     True
1     True
2    False
dtype: bool

We don't currently support the case= argument, but I believe it can easily be supported by calling lower() (or alternatively upper()) on both the input string Series as well as the substring before the call to contains().

[davidwendt]

This approach likely work if regex=False. The description does not indicate the intention specifically.
Be careful here with the target input string since some regex character classes are different depending on the case of the flag. Specifically \d, \s, \w vs \D, \S, \W so changing the case on the regex pattern without accounting for those would give incorrect results. https://docs.rapids.ai/api/libcudf/stable/md_regex.html