Resolve ILLink warnings on System.Private.DataContractSerialization Part 1; Json)

[joperezr]

This is part 1 of resolving the warnings for System.Private.DataContractSerialization. It is covering all of the warnings from the Json side, the next step will be to cover the Xml side.

Given the space is serialization, these changes mostly are about adding RequiresUnreferencedCode to all the stack that uses reflection to serialize/deserialize objects comming from the input.

cc: @eerhardt @krwq @vitek-karas

[ghost]

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Issue Details

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This is part 1 of resolving the warnings for System.Private.DataContractSerialization. It is covering all of the warnings from the Json side, the next step will be to cover the Xml side.

Given the space is serialization, these changes mostly are about adding RequiresUnreferencedCode to all the stack that uses reflection to serialize/deserialize objects comming from the input.

cc: @eerhardt @krwq @vitek-karas

Author:	joperezr
Assignees:	-
Labels:	area-Serialization, linkable-framework
Milestone:	-

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[joperezr]

As said above, the initial approach was to just mark everything as RequiredUnreferencedCode given the fact that this is a serializer, but due to the nature of DataContractSerialization and the fact that you have to always create the serializer passing in the expected types in the object graph, I'm checking to see if we can actually annotate this accordingly in order to make this serializer safe. I'll try a few things and update the PR if necessary

[eerhardt]

Just one suggestion to factor the Delegate GetMethod("Invoke") out into a helper method. Once that is addressed this looks good.

[ghost]

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