Add option to run templates in current AssemblyLoadContext

[ajcvickers]

Fixes #169

Happy to refactor and/or add tests as needed.

[mhutch]

I'd rather not change behavior in a point release so at the very least the existing parameterless method should preserve the behavior. However, it might not be necessary to add this overload at all.

There's already a mechanism for controlling whether an AppDomain is used - you can return null from ProvideTemplatingAppDomain, which is what default implementation does. It appears that EF has overridden it to return AppDomain.CurrentDomain. You may be able to fix it by removing this override https://github.com/dotnet/efcore/blob/e84696d990aefd28a7d74eddae0d0cf934320f9f/src/EFCore.Design/Scaffolding/Internal/TextTemplatingEngineHost.cs#L174

We could certainly improve usability by altering this method to detect if the AppDomain is AppDomain.CurrentDomain and in that case either throw an ArgumentException or use the non-AppDomain codepath: https://github.com/mono/t4/blob/release/v2.3.0/Mono.TextTemplating/Mono.TextTemplating/CompiledTemplate.cs#L85

[ajcvickers]

@mhutch ProvideTemplatingAppDomain is never called by Mono.TextTemplating when running the EF code. It looks like this is only used when FEATURE_APPDOMAINS is set, which is only on .NET Framework.

[mhutch]

Ah, sorry! Yes, there's no way to disable the use of AssemblyLoadContext. I thought its assembly loading was more robust than the AppDomain version but it looks like it has issues, so there should definitely be a way to opt out.