Refactor default converter registration to allow trimming when converters not used

[layomia]

Partly addresses https://github.com/dotnet/runtimelab/projects/1#card-49468644 and dotnet/runtime#45441.

App itself is not trimmed per expectations for .NET 6.

Changes optimized for the console app scenario.

TODO (in follow up):

• PR to address the Blazor app scenario is upcoming (needs changes to System.Net.Http.Json.dll and perhaps other changes).
• CI tests to catch size regressions to app and/or System.Text.Json
• PR/design to allow trimming of unused reflection code-paths (Reflection[Emit]MemberAccessor).

Console App

	Pre-trimmed STJ.dll (KB)	Trimmed STJ.dll (KB)	Pre-trimmed App (MB)	Trimmed App (MB)	Pre-trimmed Application .dll (KB)	Trimmed Application .dll (KB)
.NET 5 - S.T.J OOB (Before this change)	349	252	64.5	17.3	7	7
.NET 5 - S.T.J OOB (After this change)	347	170	64.5	17	7	7

Blazor App

	Pre-trimmed STJ.dll (KB)	Trimmed STJ.dll (KB)	Pre-trimmed App (MB)	Trimmed App (MB)	Pre-trimmed Application .dll (KB)	Trimmed Application .dll (KB)
.NET 5 - S.T.J OOB (Before this change)	349	243	17.8	2.41	20	20
.NET 5 - S.T.J OOB (After this change)	347	243	17.8	2.41	20	20