Add a @useSystemTextJsonConverter decorator for csharp only

[live1206]

Clear and concise description of the problem

In Azure/autorest.csharp#3469, we need to flag if a model needs specific handling of JSON converter.
So that, we would like to add a csharp only decorator @hasJSONConverter in TCGC, but with in a different namespace Azure.ClientGenerator.Core.CSharp

Checklist

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