String enum converter

[JeremyKuhne]

Adds public converter for converting enums to strings and vice-versa.

[JamesNK]

Json.NET supports EnumMemberAttribute to customize enum name values. It is a well used feature.

You don't need it in 3.0, but you will get a lot of requests for customizing enum names. Design for adding it in the future.

[JamesNK]

Check out https://github.com/JamesNK/Newtonsoft.Json/blob/master/Src/Newtonsoft.Json/Utilities/EnumUtils.cs

This is where Json.NET's enum heavy lifting logic lives. I basically build a contract of what the enum will look like for a given naming strategy.

[JeremyKuhne]

You don't need it in 3.0, but you will get a lot of requests for customizing enum names. Design for adding it in the future.

It shouldn't be a problem to layer in that functionality post 3.0. Didn't want to make it too complicated this time given how late we're getting.

[steveharter]

Please add a test that uses [JsonConverter] to specify the stringenum converter for a single property, but keeping the default converter as-is.

[JeremyKuhne]

Ok

[JeremyKuhne]

Apparently the converter attribute does not work with factories.

[steveharter]

Just for the record, the alternative here is to expose a single enum converter with a bool "TreatAsString" property. A new custom attribute deriving from [JsonConverter] would also need to be created in order to pass that bool on.

[steveharter]

Please add a test failure case that just uses Enum class as a property or return value from root (should fail).

[JeremyKuhne]

Not sure what you mean here. Use System.Enum directly?

[steveharter]

Yes something like JsonSerializer.Parse<Enum>("1") should fail gracefully (JsonException). I see we actually have a test for that -- so nevermind.

[steveharter]

LGTM - we can address any follow-up later after getting in this very important feature.

[JeremyKuhne]

Almost all of the failures were #38734. One failure was #37695.