Add support for MTP

[CharliePoole]

To support MTP we must check whether the assembly is pre-loaded. If it is, we should use that assembly in it's own load context and continue to seek dependencies in that context. If we resolve any dependencies, we must load them into that context as well.

If the assembly is not pre-loaded, we create our own AssemblyLoadContext and continue to use that as before.

We expect this change to resolve nunit/nunit3-vs-adapter#1389 and possibly other issues involving pre-loaded assemblies.

[CharliePoole]

This issue has been resolved in version 3.22.0

The release is available on:
GitHub.
NuGet packages are also available NuGet.org and
Chocolatey Packages may be found at Chocolatey.org