Make _wrap_json methods public

[denisSurkov]

This way wrap_json is easier to reuse.

This is simple proof of concept with Psycopg connector. If it looks okey, I will do the same with other connectors.

I don't want to place _wrap_json somewhere else. I think it is convenient to call it with context this is wrap json related to psycopg (or other connector).

Discussion: https://github.com/procrastinate-org/procrastinate/discussions/1479

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• [x] Tests
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• [ ] Documentation
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[denisSurkov]

@medihack , I made PoC with wrap_json. If it looks okey, I will do the same with other connectors.