Micro-optimizations for STJ generated source

[layomia]

• Consider public option for honoring runtime-provided custom converters
• Consider moving custom converter verification source to common helper
• When fetching runtime-provided customers, use a dictionary lookup (rather than for-loop) when JsonSerializerOptions.Converters count > ~15
• Make JsonSerializerContext.GetTypeInfo(Type) derived implementation based on dictionary-lookup if root-serializable type count > 64 (cf. https://github.com/dotnet/runtime/pull/81239/files#r1088396569)

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Issue Details

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• Consider public option for honoring runtime-provided custom converters
• Consider moving custom converter verification source to common helper
• When fetching runtime-provided customers, use a dictionary lookup (rather than for-loop) when JsonSerializerOptions.Converters count > ~15
• Make JsonSerializerContext.GetTypeInfo(Type) derived implementation based on dictionary-lookup if root-serializable type count > 64

Author:	layomia
Assignees:	layomia
Labels:	area-System.Text.Json
Milestone:	6.0.0