Add linker annotations for System.Text.Json

[layomia]

• Fixes System.Text.Json.Serialization.JsonStringEnumConverter isn't linker-safe #34449 (System.Text.Json.Serialization.JsonStringEnumConverter isn't linker-safe)
• Adds annotations to make simple serializer usages linker safe (e.g. deserializing most collections and POCOs with primitive members). More complex object graphs e.g. POCOs with POCO properties will probably need DynamicallyAccessedMemberTypes.RecursiveProperties/Constructors.
  • @steveharter mentions another view point/approach in [feature/dataflow] Support DynamicallyAccessedMemberKinds.RecursiveProperties linker#1087 (comment).

Deserializing immutable collections is not linker safe - will address with #38593.

[joperezr]

I don't think I understand why you need this?

[joperezr]

I understand why you need it now. I'm not sure I agree with what we are doing here to be honest, but I'm fine if that is the direction we are going to take.

[eerhardt]

Yes, that is the direction we've decided to take. See #36656 (comment)

The other attributes in this folder already have it:

runtime/src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/DynamicDependencyAttribute.cs

Lines 17 to 22 in 8126962

	#if SYSTEM_PRIVATE_CORELIB
	public
	#else
	internal
	#endif
	sealed class DynamicDependencyAttribute : Attribute

runtime/src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/NullableAttributes.cs

Lines 9 to 14 in 8126962

	#if INTERNAL_NULLABLE_ATTRIBUTES
	internal
	#else
	public
	#endif
	sealed class AllowNullAttribute : Attribute { }

[joperezr]

I see, honestly I don't fully agree with this approach (as opposed to having them available down level) since we probably want to allow library developers that target netstandard or < .NET5.0 to be able to annotate their libraries as well, specially if these will be libraries used for blazor wasm. Given the attribute implementation is fully netstandard2.0 compatible, I would opt instead for having compatibility packs (similar to what we did with AsyncInterfaces) for shipping the attributes down level so that we a) don't have to compile these attributes internally on a bunch of libraries and b) allow third party library writters to consume them. I don't want to push to much on doing this but just wanted to share my opinion 😄 cc: @stephentoub

[eerhardt]

LGTM

[safern]

Merging per @layomia's request.