Mitigate JsonObject performance regression.

[eiriktsarpalis]

Makes the following changes:

1. Makes the JsonValueKind computation for JsonValue instances backed by primitives a lazy operation.
2. Updates the JsonObject indexer implementation so that duplicate lookups aren't performed in the common case where a property isn't being overwritten.

Should be backported to .NET 9. Fix #107869.

Benchmarks

Using dotnet/performance#4463 on arm64.

Method	Branch	Mean	Error	StdDev	Median	Min	Max	Ratio	RatioSD	Gen0	Allocated	Alloc Ratio
Create_JsonBool	.NET 8	266.5 ns	3.24 ns	3.03 ns	266.1 ns	262.5 ns	272.6 ns	1.00	0.02	0.4779	2000 B	1.00
Create_JsonBool	PR	277.2 ns	2.87 ns	2.54 ns	277.7 ns	273.5 ns	282.0 ns	1.04	0.01	0.4781	2000 B	1.00
Create_JsonNumber	.NET 8	268.0 ns	3.33 ns	3.11 ns	268.9 ns	260.5 ns	271.3 ns	1.00	0.02	0.4773	2000 B	1.00
Create_JsonNumber	PR	285.9 ns	15.21 ns	17.52 ns	275.6 ns	269.9 ns	328.5 ns	1.07	0.06	0.5732	2400 B	1.20
Create_JsonString	.NET 8	366.3 ns	3.87 ns	3.62 ns	366.9 ns	361.1 ns	372.9 ns	1.00	0.01	0.5738	2400 B	1.00
Create_JsonString	PR	281.5 ns	2.38 ns	1.98 ns	280.8 ns	278.4 ns	286.1 ns	0.77	0.01	0.6689	2800 B	1.17
Create_JsonArray	.NET 8	486.5 ns	6.29 ns	5.89 ns	486.5 ns	477.5 ns	495.4 ns	1.00	0.02	0.7263	3040 B	1.00
Create_JsonArray	PR	463.9 ns	4.96 ns	4.64 ns	463.1 ns	455.2 ns	471.9 ns	0.95	0.01	0.7258	3040 B	1.00
Create_JsonObject_Small	.NET 8	236.1 ns	1.31 ns	1.16 ns	236.1 ns	233.7 ns	237.8 ns	1.00	0.01	0.1618	680 B	1.00
Create_JsonObject_Small	PR	198.7 ns	2.13 ns	1.99 ns	198.2 ns	195.9 ns	202.0 ns	0.84	0.01	0.2503	1048 B	1.54
Create_JsonObject_Large	.NET 8	639.4 ns	6.81 ns	6.37 ns	638.6 ns	631.1 ns	652.8 ns	1.00	0.01	0.5612	2352 B	1.00
Create_JsonObject_Large	PR	393.0 ns	2.78 ns	2.60 ns	393.0 ns	389.8 ns	397.8 ns	0.61	0.01	0.5102	2136 B	0.91

[stephentoub]

Revert delayed JsonValueKind derivation

How much of the improvement came from this? There are ways to make it safe.

[eiriktsarpalis]

Revert delayed JsonValueKind derivation

How much of the improvement came from this? There are ways to make it safe.

Not a huge amount. I figured I would rather take the regression for the sake of simplicity.

[eiriktsarpalis]

/backport to release/9.0

[github-actions]

Started backporting to release/9.0: https://github.com/dotnet/runtime/actions/runs/10940503124