Add .NET Standard 2.0 support

[Salgat]

This allows .NET Framework and older .NET Core versions (such as .NET Core 2.2) to be supported (including addressing #181).

Since much of the existing compiler directives for .NET Core 3.1 also applied to .NET Standard 2.0, I combined them (the .NET Core 3.1 test project now references the .NET Standard 2.0 version of the EventStore libraries). Unfortunately a few things had to be changed such as removing the newer range syntax. Passing all github action tests on my local branch PR.

[thefringeninja]

We cannot accept this PR at this time. We removed support for net48 as there was an issue around streaming calls being open while we requested channel shutdown: a deadlock occurs when disposing the client.

In addition, development on Grpc.Core is in maintenance mode.

Using the netstandard2.0 target should mean that grpc-dotnet is used but there are additional issues:

• Your PR only tests this target on netcoreapp3.1. It would have to test on net48 as well.
• A netstandard2.0 build would have to use WinHttpHandler to ensure it worked on Windows.
• WinHttpHandler is not available on Linux so these tests would have to run on a Windows VM.
• Running docker commands from a Windows VM is currently not supported.

Our tests spin up a new container per test fixture to ensure deterministic tests on the $all stream. It might be possible to rewrite all the tests to use a single container instance via service containers.

Lastly, there are additional restrictions around WinHttpHandler:

• Windows Server 2022 or later.
• No client streaming or bi directional calls.

Without these types of calls neither appending to streams nor persistent subscriptions would work. So this PR would have to both introduce unary replacements for these calls and patch ESDB to implement them.

[Salgat]

Looking at the gRPC requirements, it does seem that HTTP/2 is a hard blocker for older support. Thank you for the detailed explanation João!