[C++] Add SparseTensor.ToTensor() method

[asfimport]

We have functionality to convert (dense) tensors to sparse tensors, but not the other way around. Also see discussion.

Reporter: Rok Mihevc / @rok
Assignee: Rok Mihevc / @rok
Watchers: Rok Mihevc / @rok

PRs and other links:

• GitHub Pull Request #5539

_{Note: This issue was originally created as ARROW-6624. Please see the migration documentation for further details.}

[asfimport]

Kenta Murata / @mrkn:
Issue resolved by pull request 5539
#5539