Add a __dataframe__ method to the protocol dataframe object

[rgommers]

This will align the implementation with those in other libraries, xref data-apis/dataframe-api#80.

Cc @shwina

Checklist

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

[GPUtester]

Can one of the admins verify this patch?

Admins can comment ok to test to allow this one PR to run or add to allowlist to allow all future PRs from the same author to run.

[shwina]

ok to test

[raydouglass]

ok to test

[codecov]

Codecov Report

Base: 86.39% // Head: 86.42% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (55819dc) compared to base (7b0d597).
Patch coverage: 75.00% of modified lines in pull request are covered.

❗ Current head 55819dc differs from pull request most recent head 01e0607. Consider uploading reports for the commit 01e0607 to get more accurate results

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.10   #11692      +/-   ##
================================================
+ Coverage         86.39%   86.42%   +0.02%     
================================================
  Files               145      145              
  Lines             23014    23016       +2     
================================================
+ Hits              19883    19891       +8     
+ Misses             3131     3125       -6     

Impacted Files	Coverage Δ
python/cudf/cudf/io/orc.py	93.02% <ø> (ø)
python/cudf/cudf/io/parquet.py	90.83% <ø> (ø)
python/cudf/cudf/utils/ioutils.py	79.47% <ø> (ø)
python/cudf/cudf/core/df_protocol.py	88.17% <50.00%> (-0.28%)	⬇️
python/cudf/cudf/core/dataframe.py	93.77% <100.00%> (+0.04%)	⬆️
python/dask_cudf/dask_cudf/io/orc.py	91.04% <100.00%> (ø)
python/cudf/cudf/core/column/string.py	88.53% <0.00%> (+0.12%)	⬆️
python/cudf/cudf/core/groupby/groupby.py	91.22% <0.00%> (+0.21%)	⬆️
python/cudf/cudf/core/column/numerical.py	95.90% <0.00%> (+0.29%)	⬆️
... and 2 more

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[shwina]

@gpucibot merge