[FEA] Add strings support to cudf::gather

[jrhemstad]

Is your feature request related to a problem? Please describe.
cudf::gather should support gathering columns of strings.

Describe the solution you'd like
Now that the internal strings column gather function is complete, it should be plugged into the cudf::gather public API to add support for gathering strings columns.

This should be added via SFINAE of the column_gatherer function object to add a code path for string_view that calls the internal string column gather.

Note that the bitmask from the string column gather is uninitialized, and it should be updated in the gather_bitmask_kernel that same as all of the rest of the columns.