Add support for using JSON Path in SimpleURL Harvester

[tobias-hotz]

This PR adds support for using JSON Paths to get the identifier of a record.
We currently harvest a datasource where the true uuid is hidden in an array, and a JSON Pointer is not sufficient to reliably find the UUID. With this PR, the default is still JSON Pointers, but you can also switch to JSON Paths using a simple drop down menu.
I've tested this locally to confirm it works for our usecase.
My first attempt was to attempt parsing as JSON Pointer, and if that fails, fall back to JSON Path parsing. But an invalid JSON Pointer that should just fail would in that case be interpreted as a JSON Path, which is not always wanted. Also, it is not very obvious to users that JSON Paths are possible. Additionally, we always need to parse twice, which causes additional runtime overhead. So I decided to just use an explicit dropdown.

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