.NET Core 3.0 Runtime

[Shawn Black <en...@yahoo.com.INVALID>]

Howdy, all!
It's been awhile since I've done anything with OpenWhisk, but since Microsoft released .NET Core 3.0 at the end of September, I figured I'd go ahead and get the .NET Core runtime repo updated to support it.
The following PR has a handful of updates ...
Dotnet 3.0 by shawnallen85 · Pull Request #20 · apache/openwhisk-runtime-dotnet
First, obviously, is support for .NET Core 3.0, including unit tests.
Second is migrating quick starts to exist under each runtime, as some variances do exist.
Third is sharing some of the code for the test libraries as what is being tested is essentially the same.
Fourth is updating the Docker image references for .NET Core 2.2 and 3.0 -- Microsoft hosts them on their own container repo now.
This PR should also resolve the documentation discrepancy listed in issue #10.
I think that covers everything.
Please let me know if you have any questions.
Thanks!!Shawn