[C#] Add NuGet support

[asfimport]

We should add the metadata to the .csproj so we can create a NuGet package without changing any source code.

Also, we should add any scripts and documentation on how to create the NuGet package to allow ease of creation at release time.

Reporter: Eric Erhardt / @eerhardt
Assignee: Eric Erhardt / @eerhardt

PRs and other links:

• GitHub Pull Request #3891

_{Note: This issue was originally created as ARROW-4839. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 3891
#3891