pogs: add unknown field support

[zombiezen]

pogs structs should be able to insert and extract data from a struct that it does not have an explicit field for (whether that be from an omitted struct field or receiving a newer copy of the struct). This would enable read-modify-write scenarios while still preserving unknown fields.

Based on a conversation with Kenton, the way this ought to work is to allow a field that has a capnp.Struct pointer back to the original data, and then on Insert, read from it to scoop up any unknown fields.