[C#] Add .sln file and minor .csproj fix ups #20994
Description
There is currently no .sln file in the repo, which makes it hard to use the src and test code at the same time.
Also, there are some settings in the .csproj that can be moved up to the outer PropertyGroup, and not under a "Configuration|Platform" conditional, like they were in the old .csproj format.
Reporter: Eric Erhardt / @eerhardt
Assignee: Eric Erhardt / @eerhardt
PRs and other links:
Note: This issue was originally created as ARROW-4435. Please see the migration documentation for further details.