fix(masking): fixes multi-level nested fields, and hides non-JSON entries

[jamfor352]

Hi! Follow up to previous PR as I realised there were a few small issues after deploying to another environment:

1. Returning the original values if it isn't JSON-formattable is actually dangerous, as it means if the schema registry is down or if the wrong schema registry is selected in AKHQ in a multi-schema-registry-setup, the binary format data will still show as a String which in most cases still displays some of the sensitive data. To be safe, this means json_mask_by_default will now ONLY show structured data and anything else will display a placeholder message. This prevents any unwanted data leakage.

2. Fixes multi-level nested objects for the 'mask by default' option. Before we weren't prepending currentKey. So for example given a request to expose one.two.three, one.two.three would still be masked - the filter would have needed to specify just two.three (which is incorrect and would also mean another.two.three would also be unmasked). This fixes it to work as expected, so one.two.three unmasks one.two.three and also isn't applied to another.two.three

[jamfor352]

Hey @AlexisSouquiere could I get a review on this one please? A much smaller changeset which only fixes the issues described so hopefully should be a lot faster to get in than the previous PR!

[AlexisSouquiere]

Good for me. Thanks for the clarification

[jamfor352]

Good for me. Thanks for the clarification

Thank you!
cc: @tchiotludo I won't be making more changes to this branch so feel free to merge whenever suits :)