Efficient upsert of exposures in multi-paged election

[gpestana]

Currently, the implementation in #6034 of upserting exposures when the election supports are fetched in a multi-page way, the whole exposure page is read from storage.

Figure out a way to append up to n exposures to a paged exposure type without having to read the whole struct from memory. This will most likely require to implement a new sp_io primitive.