Add cudf::explode_position

[hyperbolic2346]

This PR adds support for pos_explode in cuDF. It is very similar to explode, but includes a column with the index of the array that was exploded on each row.

a                b
[0, 1]          100
[2]             200
[3, 4, 5]       300

exploded on column a would result in

a         pos        b
0         0         100
1         1         100
2         0         200
3         0         300
4         1         300
5         2         300

partially fixes #6151

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[davidwendt]

Looks good. Check the copyright year includes 2021 in any the files you changed here.

[hyperbolic2346]

@gpucibot merge