Fix and add tests for non-record payloads

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Atm unions with string payloads don't correctly roundtrip in Equinox.Cosmos with the UnionEncoder.

Also need to validate round-tripping of tupled union cases and/or whether it's sensitive to the order of the [named] elements in the tuple (as https://github.com/jet/jet.jsonnet.converters isnt)

cc @jakzale

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@jakzale Root issue was some placeholder code was stubbing out the serialization of all but records (TypeShape's UnionEncoder only added support for non-record Event Payloads in v5 so it was a non-issue at that point).

I've disabled the support for this based on:

• primitive types make a very bad payload for an event as they can't be extended easily or cleanly (people will do some very ugly String.Split stuff!)
• getting dates and strings to work correctly is non-trivial for a variety of reasons, not least the idiosyncratic decision making that Newtonsoft.Json brings

The tests are in place to enable adding clean support for this in the course of addressing #79 should it prove viable to do easily (but note the lack of extensibility all for a very minor saving in terms of typing and/or storage bytes).