Convert Column Name to String Before Using Struct Column Factory

[isVoid]

Closes #10155

build_struct_column requires that the field names to be strings. But dataframe column names can be any hashable types. Passing in column names as field names in to_struct is thus unsafe. This PR adds a check and raise a warning if the cast to string is required to take place.

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[shwina]

Maybe we should add a user warning that an implicit conversion is happening here?

[isVoid]

Offline with @shwina , what we decided here is not to implicitly cast the column names to strings. Instead, every user who calls build_struct_column factory should be aware that struct column field names can only be strings and it's caller's responsibility to cast the names to string before using this function. I will make that explicit in the docstring and update to_struct with that.

Code changed significantly since last review.

[skirui-source]

Other than Bradley's suggestions LGTM!

[isVoid]

@gpucibot merge