add netstandard2.0 support

[jano-petras]

Currently https://www.nuget.org/packages/AWS.EncryptionSDK, as well as https://www.nuget.org/packages/AWS.Cryptography.EncryptionSDK does not support netstandard2.0 target framework, which would make it usable from both .NET Framework and .NET core - while supporting .NET Framework 4.5.2+ and virtually all .NET core versions.

Is it possible to publish these packages with netstandard2.0 support? We are looking at using it in legacy .NET Framework 4.8 components.

[sunny-somani]

@texastony, do you have any updates on this? I can see the changes were merged to support netstandard2.0 but then reverted.