Add cudf::index_of_first_set_bit API

[davidwendt]

Description

Adds new bitmask API

size_type index_of_first_set_bit(bitmask_type const* bitmask,
                                   size_type start,
                                   size_type stop,
                                   rmm::cuda_stream_view stream);

Returns the index of the first valid bit in a null mask array.

Reference: #20556 (comment)

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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