Create a .NET 8.0 build of the engine

[CharliePoole]

This is needed so we and others can write clients under .NET 8.0. Currently, a .NET 8.0 client uses the netstandard 2.0 build of the engine, which lacks all the features needed to resolve dependencies. This lack seems to be the source of a number of bugs which I hope we will be able to resolve as a result.

An experimental build of the engine is currently avaliable in our MyGet feed, version 3.18.3-dev00006 and later.

[CharliePoole]

Question for discussion: I'm currently building an the engine for .NET 6.0 as well in an experimental build. It's another build with no differences from the .NET 8.0 build so I would propose to maintain it and have the .NET 6.0 NetCore runner use it for as long as we maintain that runner. The .Net 6.0 runner currently uses the NetStandard2.0 engine, which is not very satisfactory.