[REVIEW] Fix contains check in string column

[galipremsagar]

Fixes: #8832

This PR fixes contains check in the StringColumn. We were using f"^{item}$" to generate a regex and do a contains_re to check for an exact match for item in the StringColumn, but this approach would break if item by itself has some regex special characters, so replaced these checks with libcudf.search.contains which does the exact check for item in the StringColumn.

[galipremsagar]

Thanks @rgsl888prabhu & @charlesbluca for a quick review!

[galipremsagar]

@gpucibot merge

[quasiben]

rerun tests

[codecov]

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