Add cudf::strings::split_part API

[davidwendt]

Description

Adds new strings API

std::unique_ptr<column> split_part(
  strings_column_view const& input,
  string_scalar const& delimiter,
  size_type index,
  rmm::cuda_stream_view stream,
  rmm::device_async_resource_ref mr);

This will perform a split on each row using the given delimiter but only return the token at 0-based index.

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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[davidwendt]

/ok to test

[davidwendt]

/ok to test

[davidwendt]

/ok to test

[PointKernel]

One small question otherwise LGTM

[bdice]

CMake approval. (Skimmed C++, also seems fine.)

[davidwendt]

/merge