create interface for integrating third-party json parsers

[lugehorsam]

The only complicated thing here is that some parsers (e.g., RapidJson) require member annotations. For any users requiring custom member annotations, the consideration would be whether to add some sort of "member annotation" hook to plug into when doing the codegen. Aside from that, this interface should do the trick.

[lugehorsam]

@novabyte ready for another look

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