Fix SystemTextJson serializer to allow custom json converters for any data type

[Shane32]

Fixes #2257 for System.Text.Json and allows any registered JsonConverter to work properly.

It comes at a cost of an additional 25-50% of serialization time. For the "middle" query below, this represents a half of a millionth of a second of processing time. Even for the introspection query, it's only 45 millionths of a second (45μs / 0.045ms). I believe that the benefits here are worth the infinitesimal speed loss.

"Alt" is the new code

Method	Code	Mean	Error	StdDev	Gen 0	Allocated
SystemTextJson	Introspection	79,349.3 ns	159.36 ns	141.27 ns	-	161 B
SystemTextJsonAlt	Introspection	124,444.6 ns	467.13 ns	436.95 ns	-	168 B
SystemTextJson	Middle	1,442.3 ns	2.38 ns	1.98 ns	0.0229	152 B
SystemTextJsonAlt	Middle	2,075.8 ns	3.94 ns	3.68 ns	0.0229	152 B
SystemTextJson	Small	397.0 ns	1.11 ns	0.98 ns	0.0238	152 B
SystemTextJsonAlt	Small	535.5 ns	2.46 ns	2.18 ns	0.0238	152 B

[sungam3r]

Interesting change 😄 . Any test to verify decimal bug fixed?

[Shane32]

Interesting change 😄 . Any test to verify decimal bug fixed?

I'll do that.

[codecov-commenter]

Codecov Report

Merging #3189 (25bb1bf) into master (23e0fff) will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #3189      +/-   ##
==========================================
+ Coverage   84.41%   84.43%   +0.01%     
==========================================
  Files         371      371              
  Lines       16106    16034      -72     
  Branches     2605     2600       -5     
==========================================
- Hits        13596    13538      -58     
+ Misses       1884     1873      -11     
+ Partials      626      623       -3     

Impacted Files	Coverage Δ
...hQL.SystemTextJson/ExecutionResultJsonConverter.cs	89.28% <66.66%> (+2.74%)	⬆️
src/GraphQL/Types/Schema.cs	79.90% <0.00%> (+0.91%)	⬆️

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