Add System.Text.Json to JSON serializer benchmarks

[eiriktsarpalis]

Adds System.Text.Json (reflection & source gen) to the benchmark suite comparing .NET JSON serializers. Here's a few representative results using IndexViewModel values:

String Serialization

Method	Mean	Error	StdDev	Median	Min	Max	Gen0	Gen1	Allocated
SystemTextJson_Reflection	15.04 μs	0.832 μs	0.924 μs	14.82 μs	13.79 μs	16.71 μs	2.4789	0.1771	24.85 KB
SystemTextJson_SourceGen	10.51 μs	0.114 μs	0.096 μs	10.49 μs	10.35 μs	10.65 μs	2.4715	-	24.55 KB
JSON.NET	33.38 μs	1.074 μs	1.149 μs	33.09 μs	31.62 μs	36.20 μs	5.9487	1.1154	59.33 KB
Jil	39.15 μs	1.095 μs	1.172 μs	38.73 μs	37.49 μs	42.10 μs	5.7115	0.9791	56.7 KB
Utf8Json	26.08 μs	1.140 μs	1.313 μs	25.71 μs	24.51 μs	28.99 μs	2.4222	0.1938	24.55 KB

String Deserialization

Method	Mean	Error	StdDev	Median	Min	Max	Gen0	Gen1	Allocated
SystemTextJson_Reflection	24.53 μs	0.437 μs	0.388 μs	24.65 μs	23.93 μs	25.31 μs	2.1980	0.2867	22.01 KB
SystemTextJson_SourceGen	24.23 μs	0.464 μs	0.388 μs	24.10 μs	23.82 μs	25.00 μs	2.1654	0.2953	22.01 KB
JSON.NET	53.29 μs	2.270 μs	2.614 μs	53.07 μs	50.14 μs	58.43 μs	3.0352	0.4047	31.31 KB
Jil	41.49 μs	0.542 μs	0.453 μs	41.63 μs	40.56 μs	42.18 μs	2.3217	0.1658	23.49 KB
Utf8Json	39.39 μs	2.146 μs	2.471 μs	39.98 μs	36.40 μs	42.61 μs	3.3665	0.2927	33.87 KB

Stream Serialization

Method	Mean	Error	StdDev	Median	Min	Max	Gen0	Allocated
DataContractJsonSerializer	81.58 μs	4.635 μs	5.337 μs	81.90 μs	75.71 μs	93.49 μs	-	2416 B
SystemTextJson_Reflection	14.04 μs	0.648 μs	0.746 μs	13.93 μs	12.99 μs	15.38 μs	-	464 B
SystemTextJson_SourceGen	14.05 μs	0.858 μs	0.989 μs	13.69 μs	13.06 μs	16.27 μs	-	464 B
JSON.NET	33.31 μs	1.696 μs	1.953 μs	32.63 μs	30.73 μs	37.13 μs	0.1249	2448 B
Jil	40.21 μs	2.319 μs	2.670 μs	40.84 μs	36.95 μs	47.08 μs	-	96 B
Utf8Json	24.51 μs	1.040 μs	1.156 μs	24.01 μs	23.42 μs	26.92 μs	-	-

Stream Deserialization

Method	Mean	Error	StdDev	Median	Min	Max	Gen0	Gen1	Allocated
DataContractJsonSerializer	223.90 μs	2.331 μs	1.820 μs	223.84 μs	221.09 μs	226.39 μs	9.0726	1.0081	94.25 KB
SystemTextJson_Reflection	25.92 μs	1.005 μs	1.158 μs	25.56 μs	24.81 μs	28.44 μs	2.1930	0.2990	22.01 KB
SystemTextJson_SourceGen	25.27 μs	0.583 μs	0.599 μs	25.22 μs	24.32 μs	26.64 μs	2.1619	0.2948	22.01 KB
JSON.NET	56.12 μs	2.302 μs	2.651 μs	55.36 μs	53.00 μs	60.91 μs	3.4079	0.4260	34.27 KB
Jil	61.04 μs	1.036 μs	0.919 μs	60.84 μs	60.07 μs	62.32 μs	2.6918	0.2692	26.7 KB
Utf8Json	35.55 μs	0.341 μs	0.267 μs	35.55 μs	35.14 μs	35.98 μs	2.1067	0.2809	21.58 KB

Fix dotnet/runtime#78451.

[eiriktsarpalis]

Note that source gen requires C# 9 since it is setting init-only properties. I've conditioned the lang version on the target framework as it seemed like the smallest possible fix, but I defer to maintainers on the right approach.

cc @adamsitnik

[adamsitnik]

@eiriktsarpalis currently this repo has no CI leg to verify that 3.1, 6.0 and 7.0 builds are still working. I've created #2732 to address that. Once I get it merged (somewhen next week), I am going to review this PR and merge it too.

[eiriktsarpalis]

@eiriktsarpalis currently this repo has no CI leg to verify that 3.1, 6.0 and 7.0 builds are still working. I've created #2732 to address that. Once I get it merged (somewhen next week), I am going to review this PR and merge it too.

FWIW I have checked locally and it compiles for all TFMs. Given that 3.1 is going out of support next month, perhaps we could just remove support for it altogether? We can then use a consistent language version for all TFMs.

[adamsitnik]

Overall it looks good to me. Please take a look at my comments before merging the PR.

Thank you for adding more benchmarks @eiriktsarpalis !