Consider making source generated System.Text.Json Optional

[keithwill]

Consider making source generated System.Text.Json Optional

[keithwill]

This was implemented incidentally during recent work to reduce user defined type requirements. Non source generated types can be used, but they still have to be added to the VestPocketOptions before opening a store (so that VestPocket knows how to deserialize arbitrary object JSON with only a $type name propery). There is also the expectation that a JsonSerializerContext will be added to the VestPocketOptions, but we may relax that requirement in the future (allowing non source generated types to be serialized / deserialized even though a source generated context is required is an odd arrangment).