Add `beforeLoad` callback

[nezuo]

It should be possible to modify the data of a document before it loads. For example, you could implement reconciliation (think ProfileService:Reconcile) or decompression of the document.

Some open questions:

• Should this callback run before or after migration? Should there be a callback for both?
• Should this callback be global or per collection?

I'd also like to add a reconciliation example to the docs after this feature is added.

[jackTabsCode]

I was just about to suggest a feature request for transforming data. Here's what I'm envisioning for our project:

• Our in-engine representation of player data contains types we otherwise couldn't store directly in DataStores, like Vector3 and DateTime. In our codebase, we use Flamework to automatically generate our player save validator. So, our validator would expect these types to be correct when it runs.
• We can write a load transformation function that, importantly, runs before the collection validator, and converts these to their bona fide Roblox types. I.e. { DateTime: number } <-> DateTime and { Vector3: { x: number, y: number, z: number } } <-> Vector3
• In beforeSave, we can convert our Roblox types to some representation compatible with DataStores (we can do this now)

Does what I'm suggesting make sense for this issue, or should I make a new one?