Recoup some of the perf losses in cold start serialization

[eiriktsarpalis]

Makes the following changes:

• Use ConstructorInfo invocations instead of static MethodInfo when instantiating generic metadata types. In my measurements this is ~2x faster.
• Delegate JsonTypeInfo<T> construction to existing JsonConverter<T> instances, where possible.

Method	Job	Toolchain	Mean	Error	StdDev	Median	Min	Max	Ratio	MannWhitney(3%)	RatioSD	Gen 0	Gen 1	Allocated	Alloc Ratio
NewCustomConverter	Job-GMEIGR	main	24.35 us	0.312 us	0.244 us	24.37 us	23.92 us	24.78 us	1.00	Base	0.00	0.9720	0.0972	9.74 KB	1.00
NewCustomConverter	Job-XOBSON	PR	19.80 us	0.389 us	0.400 us	19.78 us	19.23 us	20.45 us	0.82	Faster	0.02	0.7650	0.0765	7.86 KB	0.81

Fix #73388.

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Issue Details

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Reflection-based initialization of the generic metadata types was being done via generic MethodInfo invocations, which can be ~2x slower compared to ConstructorInfo invocations.

Method	Job	Toolchain	Mean	Error	StdDev	Median	Min	Max	Ratio	MannWhitney(3%)	RatioSD	Gen 0	Gen 1	Allocated	Alloc Ratio
NewCustomConverter	Job-CBOUFE	main	27.58 us	1.400 us	1.556 us	27.92 us	24.88 us	30.32 us	1.00	Base	0.00	0.8943	0.0994	9.74 KB	1.00
NewCustomConverter	Job-NMGPDV	PR	24.55 us	1.478 us	1.702 us	24.16 us	21.94 us	27.43 us	0.89	Faster	0.09	0.8059	0.0895	8.52 KB	0.88

Fix #73388.

Author:	eiriktsarpalis
Assignees:	eiriktsarpalis
Labels:	area-System.Text.Json
Milestone:	7.0.0