Newtonsoft.Json -> System.Text.Json

[beppemarazzi]

First quick and dirty attempt to migrate from Newtonsoft.Json to System.Text.Json (see #125), strictly related and partially dependent on serilog/serilog-formatting-compact-reader#37

@nblumhardt: Please consider this PR as a reference only! WARNING, WARNING, WARNING!!!!! in this case the only "test" i did was with SeqQuery and SeqTail tools, and both them SEEMS to work...

Warn also in this point (this is why AppVeyor fails to build):

seq-api/example/SeqTail/SeqTail.csproj

Line 16 in c7c0bef

<PackageReference Include="Serilog.Formatting.Compact.Reader" Version="3.0.0-dev-local" />

where i "redirected" the dependency to my local nuget feed because i want to use serilog-formatting-compact-reader with serilog/serilog-formatting-compact-reader#37 applied