Downgrade to .NET Standard 2.0

[unaizorrilla]

At this moment, Esquio is .NET Standard 2.1 with this restrictions to 2.0

• ValueTask
• Performance Counters and EventSource code on diagnostics

Some people ask about use Esquio on .NET Standard 2.0 to allow full framework and other not up to date projects. We can create a multitarget package to allow both ( .NET Standard 2.0 and .NET Standard 2.1 )

Here is some info about multi-target libraries on .NET Core

• https://docs.microsoft.com/en-us/nuget/create-packages/multiple-target-frameworks-project-file
• https://medium.com/@toptensoftware/non-trivial-multi-targeting-with-net-7e112f6fd7f2
• https://www.bugsnag.com/blog/building-multi-targeted-dot-net-libraries
• https://markheath.net/post/csproj-conditional-references

[unaizorrilla]

Branch features/MultiTarget is the working branch for this feature!!