Handle missing weekend stock prices in sync process

[zachgoll]

There are a few different areas that we could potentially solve this issue:

1. At the provider level (not a good option since we can't always control what the provider returns)
2. At the base model level (i.e. Security::Price.find_prices)
3. At the sync model level (i.e. Account::Holding::Syncer)

To keep the gapfill logic isolated from the base model, I've added a Gapfiller class that operates on the fetched prices directly in the sync process.