Introduce JsonPathString

[xzfc]

Part 1/3 of #3637 (comment).

Roadmap/general idea: let the new and the old implementation coexist for a while. Introduce a new trait that both implementations will implement. Use the same set of unit tests for both implementations.

This PR adds a new trait JsonPathInterface and its implementation JsonPathString which behaves exactly like the old implementation.
All the code is updated to use the new interface. These are mostly mechanical changes in tests.

Additionally, this PR adds an empty stub implementation of JsonPathV2 just to check that everything compiles if we replace the old implementation with the new one. It panics though.

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[timvisee]

With a trait like this it seems a bit like over engineering, but I get that we only do this during the migration period. So then I'd be fine to keep it like this 👍

[generall]

Please separate implementation and integration next time. 1000+ lines is still too big