System.Text.Json support.

[Quahu]

Continued from #100

Because of the overall better performance and no unnecessary LINQ allocations System.Text.Json will replace Newtonsoft.Json in the future. Currently, Newtonsoft.Json is still the default and the serializer service has to be replaced in order to use System.Text.Json:

services.Replace(ServiceDescriptor.Singleton<IJsonSerializer, SystemJsonSerializer>());

Important

SystemJsonSerializer is only available if your application targets .NET 8 and newer.

Some benchmarks

From top to bottom are in order:

• an older Disqord build (.NET 6) from NuGet
• a Disqord build (.NET 8) from this branch
• a Disqord build (.NET 8) using System.Text.Json from this branch

Old Newtonsoft (.NET 6) Build

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
RestApiCallLargeBodyHighLevel	1,655.71 us	31.671 us	40.054 us	46.8750	46.8750	46.8750	754 KB
RestApiCallLargeBodyLowLevel	1,662.00 us	7.340 us	6.129 us	46.8750	46.8750	46.8750	746 KB
RestApiCallEmptyBodyHighLevel	25.74 us	0.504 us	0.656 us	0.1831	-	-	10 KB
RestApiCallEmptyBodyLowLevel	23.84 us	0.283 us	0.264 us	0.1831	-	-	10 KB
WebSocketLifecycleLargeGuildCreate	930,109.53 us	18,475.223 us	32,839.689 us	8000.0000	4000.0000	1000.0000	435,414 KB

New Newtonsoft (.NET 8) Build

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
RestApiCallLargeBodyHighLevel	1,173.730 us	19.3825 us	18.1304 us	46.8750	46.8750	46.8750	904 KB
RestApiCallLargeBodyLowLevel	1,231.455 us	22.5396 us	36.3973 us	46.8750	46.8750	46.8750	897 KB
RestApiCallEmptyBodyHighLevel	8.632 us	0.4001 us	1.1609 us	0.1831	-	-	9 KB
RestApiCallEmptyBodyLowLevel	5.837 us	0.1155 us	0.2169 us	0.1831	-	-	9 KB
WebSocketLifecycleLargeGuildCreate	735,406.145 us	14,690.9525 us	42,386.7569 us	9000.0000	8000.0000	1000.0000	460,100 KB

System.Text.Json (.NET 8) Build

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
RestApiCallLargeBodyHighLevel	768.534 us	20.5097 us	60.1514 us	46.8750	46.8750	46.8750	402 KB
RestApiCallLargeBodyLowLevel	714.988 us	8.9789 us	8.3989 us	46.8750	46.8750	46.8750	394 KB
RestApiCallEmptyBodyHighLevel	3.007 us	0.0492 us	0.1070 us	0.0725	-	-	4 KB
RestApiCallEmptyBodyLowLevel	3.580 us	0.0712 us	0.0926 us	0.0687	-	-	3 KB
WebSocketLifecycleLargeGuildCreate	192,896.364 us	3,717.7621 us	3,295.6993 us	-	-	-	156,412 KB