Create a new Activout.RestClient.Test.Json project to run the same tests against both JSON implementations

[twogood]

Create a new Activout.RestClient.Test.Json project with xunit. Do not modify any existing code.

Depend on both Activout.RestClient.Json and Activout.RestClient.Newtonsoft.Json

Run exactly the same JSON-dependent test cases (based on RestClientTests.cs, SimpleValueObjectTest.cs etc that are not actually tied to a specific JSON implementation) (using [Theory] or similar) with both JSON implementations of Activout.RestClient.Json and Activout.RestClient.Newtonsoft.Json.

This requires having separate CreateRestClientBuilder() implementations for the different JSON support implementations.