Part of #4889

To better support writing nested data, here we update our builtin StructLayout in a backwards-compatible fashion.

• StructStrategy will shred struct fields into their own StructLayout recurseively
• StructLayout can support nullable structs now. It does this by writing a new child layout containing the validity buffer for nullable arrays

I use the RealNest dataset to evaluate, which contains a copy of ~200k github pull request webhook events. Nested struct layout reduces file size over the previous strategy by about ~10%, and also makes pushdown into the nested columns possible.

Some open questions

• The validity child requires some extra handling. It seems like the validity handling is very dependent on the expression being pushed down. For example if I'm doing a simple project of a child field, then adding the validity to the result is a simple masking operation. If I'm pushing down an UNNEST or something else that increases the result size, it is hard to map the validity buffer onto the projection_eval result
• I collect all validity chunks into a single buffer at write time. The idea being that it's better to access the struct validity as a single unit since it is much smaller than the data size. Assuming an 8MB target segment size, this lets us comfortably fit ~64mm rows into a single segment. Another alternative is to bring back roaring, or enable some other boolean compressors.