System.Text.Json support.

[Quahu]

Continued from #100

Because of the overall better performance and no unnecessary LINQ allocations System.Text.Json will most likely replace Newtonsoft.Json when feature parity is achievable (.NET 7 or 8 hopefully).

System.Text.Json:

Note: performance and allocations will differ in the future as the current Disqord implementation isn't complete, so take these results with a grain of salt.

Method	Job	UnrollFactor	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
RestApiCallLargeBodyHighLevel	DefaultJob	16	2,124.95 us	40.635 us	39.909 us	93.7500	46.8750	46.8750	461 KB
RestApiCallLargeBodyLowLevel	DefaultJob	16	2,114.51 us	30.378 us	26.930 us	117.1875	58.5938	46.8750	453 KB
RestApiCallEmptyBodyHighLevel	DefaultJob	16	10.91 us	0.177 us	0.236 us	1.3733	-	-	4 KB
RestApiCallEmptyBodyLowLevel	DefaultJob	16	10.83 us	0.093 us	0.087 us	1.3123	-	-	4 KB
WebSocketLifecycleLargeGuildCreate	Job-TAXHVY	1	434,255.21 us	3,697.672 us	3,277.890 us	2000.0000	1000.0000	-	120,936 KB
WebSocketLifecycleSmallGuildCreate	Job-TAXHVY	1	7,077.23 us	139.918 us	181.933 us	-	-	-	1,086 KB

Newtonsoft.Json:

Method	Job	UnrollFactor	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
RestApiCallLargeBodyHighLevel	DefaultJob	16	2,878.45 us	29.372 us	27.474 us	218.7500	62.5000	46.8750	857 KB
RestApiCallLargeBodyLowLevel	DefaultJob	16	2,960.26 us	59.181 us	95.567 us	218.7500	54.6875	46.8750	850 KB
RestApiCallEmptyBodyHighLevel	DefaultJob	16	14.30 us	0.234 us	0.351 us	3.3264	-	-	10 KB
RestApiCallEmptyBodyLowLevel	DefaultJob	16	14.20 us	0.276 us	0.329 us	3.2349	-	-	9 KB
WebSocketLifecycleLargeGuildCreate	Job-SFWGZJ	1	1,425,774.56 us	23,537.066 us	27,105.321 us	54000.0000	20000.0000	3000.0000	322,344 KB
WebSocketLifecycleSmallGuildCreate	Job-SFWGZJ	1	5,178.65 us	300.440 us	885.854 us	-	-	-	1,990 KB

Benchmarked using https://github.com/kcut/institute-of-performance-research.