Add support for System.Text.Json

[Astn]

Why?

• Performance
• System.Net.Json is a built in

See docs
https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to

[Astn]

I've pushed a branch up that starts work on this. Feel free to collaborate on it with me.
branch: https://github.com/Astn/JSON-RPC.NET/tree/SwitchToSystem.Text.Json

[Astn]

Initial tests show this branch is almost 2x as fast the current approach.

Starting benchmark
processed 50 rpc in      0ms for         ∞ rpc/sec
processed 100 rpc in     0ms for         ∞ rpc/sec
processed 300 rpc in     0ms for         ∞ rpc/sec
processed 1,200 rpc in   3ms for         400,000.00 rpc/sec
processed 6,000 rpc in   11ms for        545,454.55 rpc/sec
processed 36,000 rpc in          83ms for        433,734.94 rpc/sec
processed 252,000 rpc in         686ms for       367,346.94 rpc/sec
Finished benchmark...