ASP.NET Core Support

[kraksyy]

ASP.NET core is a high performance web framework that by default uses Kestrel web server written in C# while it's decent, having a even more optimized web server could be beneficial in scenarios that require high amount of data throughput while being cost effective to pull off as in developer cost and time (It is easier to find cheap and more C# developers than get Go developers and handle more data than IO oriented dev environments like Node.JS or Python).

This issue will be used to track a pull request I'm working on In spare time to implement C# support into Nginx Unit.
Not only this will increase performance of ASP.Net applications who focus on raw processing speed but also allow C# applications to provide a lightweight web service without bringing in a fully fledged backend framework.

Currently a C# web service can be hacked together through use of "libnginxunit" but this wouldn't have benefits of having a dedicated support.

Follow up of: #565