Support walking slices, recursive types & prepare for fetching secrets in batches

[VojtechVitek]

• Walk any given config recursively (support nested structs, double-indirection etc.)
• Support slices and arrays
• Return slice of fields that were collected for secret hydration -- this will allow us to fetch secrets in batches, if supported by the provider

Possible limitation: If there is a circular dependency in the given config, we might hit an endless recursion. But is that even possible for configs? Perhaps we could add a limit on the depth of recursion.