upgrade to dotnet core

[mirbeta]

项目升级.NET CORE 和standard，同时支持.NET CORE和.NET framework.

需要注意的是升级之后，原来的.NET framework最少版本变更为.NET framework 4.6

[CLAassistant]

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1 out of 2 committers have signed the CLA.

:white_check_mark: DrDao
:x: John

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