[release/3.1] Improve JsonSerializer support for derived types (#40654)

[layomia]

Ports #40654 to 3.1

Description

• When determining whether to handle a derived type as IList (i.e populating directly, without using a converter), check whether the runtime type is assignable to IList, not the declared type.
  This fixes https://github.com/dotnet/corefx/issues/40597.
• Preemptively throw NotSupportedException on deserialization of implementing types that don't have default constructors. This prevents a NullReferenceException when we try to create the instance.
• Detect and use the implemented type of abstract implementing types for (de)serialization. This allows us to correctly throw NotSupportedException on deserialization (because abstract types have no default constructor).
• Document expectations of (de)serializing collections in System.Collections.ObjectModel in tests.

Customer Impact

• Prevents NullReferenceException when trying to deserialize a derived type without a parameterless ctor (https://github.com/dotnet/corefx/issues/41242).
• Enables support for System.Collections.ObjectModel collections where applicable, throws NotSupportedException otherwise
  (https://github.com/dotnet/corefx/issues/40597).

Regression?

No.

Risk

Low. This PR does not modify already existing features, so the possibility of breaking functionality or introducing regressions is limited. Extensive test cases were added to ensure that the new changes work as expected.

[ahsonkhan]

cc @danmosemsft, @ericstj

[danmoseley]

This PR does not modify already existing features, so the possibility of breaking functionality or introducing regressions is limited

I'm not sure what 'modify existing features' means exactly - I see the changes are in existing code paths.

[danmoseley]

Thanks @layomia . I'm fine with merging this if @ericstj approves it. It's bug fix level work to address mainstream customer reported issues in JSON.

[layomia]

This PR does not modify already existing features, so the possibility of breaking functionality or introducing regressions is limited

I'm not sure what 'modify existing features' means exactly - I see the changes are in existing code paths.

@danmosemsft I meant "modify the behavior of supported (de)serialization scenarios".