JsonMapper object

[ewjoachim]

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We need an abstraction layer over json loading and dumping because:

• It's needed in connectors (and we wish we wouldn't have to set it up in every connector)
• It's needed in the CLI (for procrastinate defer)

What we think about doing:

• Create a class JsonMapper(json_loads: Optional[Callable], json_dumps: Optional[Callable])
• Store a json mapper in the App
• When building the Connector, set its json mapper (not via the constructor)
• The CLI should use app.json_mapper in defer()