Add an `UnnestBuilder` annotation

Currently, when a record references another record that is in scope for builder generation (e.g `A` has a field of type `B`), then the "outer" builder (for `A`) treats the inner record as a normal field and just generates some functional methods to assist with its construction.

In some cases however, we'd rather not do that and instead have all the methods on the inner builder (for `B`) present on the outer builder (for `A`). Ideally this would be handled efficiently by extending the class but that may require methods to no longer be `final` (which is fine as the builders themselves are `final`).