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 https://github.com/serilog/serilog-formatting-compact-reader/pull/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): https://github.com/datalust/seq-api/blob/c7c0bef9ee5684b7a043e0105629045b08fcf48c/example/SeqTail/SeqTail.csproj#L16 where i "redirected" the dependency to my local nuget feed because i want to use serilog-formatting-compact-reader with https://github.com/serilog/serilog-formatting-compact-reader/pull/37 applied