feat(orm): add loadOnce

[RomainLanz]

Hey there! 👋🏻

This PR adds the loadOnce helper.

This helper is particularly useful when you're unsure whether a relation is already loaded and want to avoid unnecessary database calls if the data is already in memory.

Currently, loadOnce does not accept a callback like load does, as it's designed to avoid altering the query.

If you need to load nested relationships, you can do so like this:

await user.loadOnce('profile')
await user.profile.loadOnce('country')
// ...