Fixed serialisation issue in samples using Newtonsoft Json.NET

It seems that during the .NET upgrade, I broke Newtonsoft Json.NET serialisation. I removed the usage of JsonConstructor and NuGet that extended Json.NET with private constructors. Unfortunately, the test suite was not strong enough. That was strengthened up in this release.

I added ContractResolver which enables private non-default constructors.

Details

• Added JsonObjectContractProvider that resolves Json object constructor, to support non-default, private constructor by default.
• Moved JSONserialisationn to dedicated project to be able to not have a dependency on Core in the Test project,
• Added default JsonSerializerSettings and used in Newtonsoft related projects,
• Added NonDefaultConstructorContractResolver and NonDefaultConstructorMartenJsonNetContractResolver that use JsonObjectContractProvider internally
• Added missing tests for Cart Confirmation and Adding Product in Marten ECommerce Sample.
• Updated Requests classes to records in Marten ECommerce Sample.

Note: JsonObjectContractProvider will be added to Marten in the follow-up PR.