Handle nested columns

[roeap]

We need to handle nested columns in various locations, data skipping being an obvious one.

We should come up with a robust and simple design that is capable of handling column mapping and other special scenarios as well.

Known problems to solve:

• [x] https://github.com/delta-incubator/delta-kernel-rs/issues/422
• [x] https://github.com/delta-incubator/delta-kernel-rs/issues/443
• [ ] https://github.com/delta-incubator/delta-kernel-rs/issues/353
• [ ] https://github.com/delta-incubator/delta-kernel-rs/issues/423
• [ ] https://github.com/delta-incubator/delta-kernel-rs/issues/442
• [x] https://github.com/delta-incubator/delta-kernel-rs/issues/444

[nicklan]

@roeap can you clarify what's not properly handled today? afaict here we will traverse into nested columns.

So is there something more general you're talking about here?