Replace Json.NET with SpanJson

[hez2010]

.NET Core 2.1 and .NET Standard 2.1 have introduced new APIs Span<T> and Memory<T> which provide high-performance memory access and operation, and SpanJson (see: https://github.com/Tornhoof/SpanJson) is a json library which uses Span<T> to accelerate the json parsing process.
Hope that aliyun-log-dotnetcore-sdk can replace Json.NET with SpanJson to improve its performance and throughput.

Note that the default behavior of serializing an Enum in SpanJson is to convert the enum value to string, which is different from Json.NET, so you may need to create a CustomFormatter (see: Tornhoof/SpanJson#112) to keep previous behavior.

Thanks!

[JasonMing]

I will consider to separate the part of json serialization and make it be able to adapt different json libraries.
But it may take a long time.

[JasonMing]

System.Text.Json is in rc stage now, I will migrate the json part to that library after they release.