Evaluate new json v2 for performance

We do a lot of json parsing that is well known to be very resource hungry in golang, to the point the language released a new experimental v2 version in https://pkg.go.dev/encoding/json/v2#Unmarshal

Once it reaches stability we should evaluate it

@liggitt has more insights on this work

Start with the linked encoding/json/v2 Unmarshal documentation and inspect how knftables currently handles JSON parsing. Once the experimental implementation is stable, compare its performance with the current approach and document whether the results justify changing it.

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