Groupby.shift c++ API refactor and python binding

[isVoid]

Closes #7183 , follow up of #7910

This PR:

• refactors existing libcudf groupby::shift API, which only takes a single column, to accept multiple columns.
• adds cython and python bindings for groupby.shift. Example python usage:

df = cudf.DataFrame({"a":[1,2,1,2,2], "b":["x", "y", "z", "42", "7"]})
>>> df.groupby("a").shift(1)
      b
a      
1  <NA>
1     x
2  <NA>
2     y
2    42

Minor refactors:

• adds use_thread parameter to dataset_generator.rand_dataframe to expose thread pool config.

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[isVoid]

Failed tests should be fixed by #8272

[cwharris]

lgtm

[cwharris]

should we change this index in the test, or should we be returning the same order as pandas in the public api?

[isVoid]

Currently groupby.agg already diverts from pandas in row orders, in that cudf does not alter the key order returned from libcudf to match pandas order. Here we follow the same convention.

[kkraus14]

@gpucibot merge

[isVoid]

rerun tests

[isVoid]

rerun tests

[kkraus14]

rerun tests

[kkraus14]

@gpucibot merge

[galipremsagar]

rerun tests