[Variant] Improve API for iterating over values of a VariantList

[alamb]

• Part of [EPIC] [Parquet] Implement Variant type support in Parquet #6736

• Related to [Variant] Consider validating variants on creation (rather than read) #7684

At the moment iterating over the fields produces an iterator of Result which is annoying to deal with and doesn't play very well with most other Rust iterator combinators (like map, for example)

This is a tracking issue for a conversation @scovich and I had about how to improve iterating over a VariantList / fields of a VariantObject

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Something I noticed here was that it would be really nice if:

1. This returned an Iterator rather than a Result
2. We could implement iter() and IntoIterator for VariantList

That would make using it more ergonomic, though it would require either panic'ing or else validating the offsets on construction 🤔

Originally posted by @alamb in #7666 (comment)