JsonSerializerOptions and Null Handling - Enhancement

[dammitjanet]

The JsonSerializer used within JsonWritableOptions does not provide any way to serialise the object with any additional JsonSerializerOptions

For example, I would find it useful to be able to set DefaultIgnoreCondition (NullHandling) to WhenWritingNull JsonSerializerOptions and I'm sure that in some cases, that the ability to add different JsonConverters would also be of use.

[dammitjanet]

I note the solution to #308 also refers to this as a solution