[REVIEW] concat(): support sub-types and object wrappers

[madsbk]

This PR makes concat() support sub-types and object wrappers of DataFrame, Series, etc.

This is motivated by rapidsai/dask-cuda#353, which wraps objects in a "proxy" class.
But the PR is generally useful for downstream projects that inherit from DataFrame, Series, etc.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[codecov]

Codecov Report

Merging #5879 into branch-0.15 will increase coverage by 0.31%.
The diff coverage is 90.90%.

@@               Coverage Diff               @@
##           branch-0.15    #5879      +/-   ##
===============================================
+ Coverage        84.12%   84.43%   +0.31%     
===============================================
  Files               80       80              
  Lines            13087    13416     +329     
===============================================
+ Hits             11009    11328     +319     
- Misses            2078     2088      +10     

Impacted Files	Coverage Δ
python/cudf/cudf/core/reshape.py	88.67% <90.90%> (-0.43%)	⬇️
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
python/custreamz/custreamz/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_orc.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_json.py	100.00% <0.00%> (ø)
...ython/dask_cudf/dask_cudf/io/tests/test_parquet.py	100.00% <0.00%> (ø)
python/cudf/cudf/utils/applyutils.py	98.75% <0.00%> (+0.02%)	⬆️
... and 31 more

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