Support .netstandard2.1

[gldeng]

No description provided.

[vizipi]

Pull request analysis by VIZIPI

Below you will find who is the most qualified team member to review your code.
This analysis includes his/her work on the code included in this Pull request, in addition to their experience in code affected by these changes ( partly found within the list of potential missing files below )   Feedback always welcome

No other active qualified developers found to review these specific changes. You might consider involving more team members with these code segments.

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Potential missing files from this Pull request

No files found with a 40% threashold :)
No commonly committed files found with a 40% threashold

[argfoo-web3]

Build will fail due to 1.3.0 for AElf.Cryptography and AElf.Types not supporting .Net Standard 2.1. We will need to make sure we create a new nuget version for this PR (AElfProject/AElf#3433) first before we merge this.