Create a struct to organize optional fields that are default_on_eof in snapshot

[AshwinSekar]

Problem

Right now when adding new fields to snapshot we append them to the end of the snapshot and deserialize them as default on eof. These fields are split between the AccountsDbFieldsToSerialize struct (which happened to originally be serialized at the end of the snapshot) + fields that are manually appended during serialization.
This is quite confusing and it would be better to keep all of these optional fields in a separate struct that is serialized at the end of the snapshot. This will also allow us to easily check the serialization dependency between these fields and avoid any situations where data is being deserialized to the wrong field.
Hopefully this will also make it easier to move these fields properly into the snapshot when we do version upgrades.

Proposed Solution

[brooksprumo]

Assigned myself as well so that I can find this issue again easily in the future.