feat: check whether structs within arrays/slices/maps implement a Marshaler interface

[Adjective-Object]

The current implementation of interface checks only handles cases where the root type implements json.Marshal. If it does not, the type is "unpacked" to a raw struct type, which is then checked for json: tags.

This fails when the root type does not implement json.Unmarshal, but is a transparent container for a type that contains a json.Marshal; e.g. []MyType or map[string]MyType where MyType implements json.Marshal.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.45%. Comparing base (a6b60d7) to head (47c706f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #87 +/- ## ======================================= Coverage 79.45% 79.45% ======================================= Files 3 3 Lines 219 219 ======================================= Hits 174 174 Misses 32 32 Partials 13 13 ```

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[tmzane]

Thanks!