Add support for IAsyncEnumerable #175

[cmxl]

Solves #175

IAsyncEnumerable is only available since netstandard2.1.
So I made the EndpointBase classes partial, added a new partial class file and removed it from compilation when the targetframework is not netstandard2.1

What I was not sure of, is how to add test for this feature.

[ardalis]

Can you add any tests or something in the sample demonstrating its usage?

[cmxl]

I just added tests and a sample endpoint.
I had to add a direct project reference though, due to automapper failing to load the correct assembly when referencing a multitargeted project. That's also why I needed to add <SetTargetFramework>TargetFramework=netstandard2.1</SetTargetFramework> to the project reference in csproj files.
Otherwise the netstandard2.0 version would have been referenced by default.

[maxkoshevoi]

Will this be an issue for NuGet consumers?

[maxkoshevoi]

Maybe it makes sense to migrate whole package to netstandard2.1. It's only intended to be used in ASP.NET Core anyway

[cmxl]

Will this be an issue for NuGet consumers?

I remember reading something about it, but did not have time to try.

Maybe it makes sense to migrate whole package to netstandard2.1. It's only intended to be used in ASP.NET Core anyway

I would personally like to upgrade to netstandard2.1.
The compatibility table is here:
https://docs.microsoft.com/en-us/dotnet/standard/net-standard?tabs=net-standard-2-1#select-net-standard-version

[maxkoshevoi]

@ardalis How about updating to netstandard2.1?

[cmxl]

@ardalis any thoughts about upgrading to netstandard2.1?

[ardalis]

Yes that's fine with me.

[cmxl]

#180 has bumped the target framework to netcore3.1, so I removed the netstandard2.1 stuff.
code is so much cleaner now :)