Add .NET Standard 2.0 support

[Metadorius]

Would it be possible for the driver to support .NET Standard 2.0? .NET Standard 2.1 is honestly not that useful compared to 2.0 because 2.0 is supported on .NET Framework 4.6.1+, and 2.1 is only supported on .NET Core and more recent version. According to the documentation by MS that version supports most of the stuff that's needed for libraries. What's not supported can be polyfilled/backported like AsyncInterfaces.

[Metadorius]

So far what I discovered:

• WebSocket.Client needs to be downgraded to 4.7.0 on .NET Standard 2.0;
• Ideally System.Threading.Channels should have async versions added into the .NS2.0 version (see Add missing Async methods to System.Threading.Channels on .NET Standard 2.0 dotnet/runtime#96039);
• Default interface implementation (thankfully there's only one and it's so insignificant that it can be not used at all);
• Some more tweaks here and there like string not able to be converted to ReadOnlySpan<char> implicitly or HTTP client not having some async methods.