Optimization: set_source and group_by_source callbacks on types that support loading through

[zachdaniel]

Right now, when an attribute supports being "loaded through", what we do is we do its load logic for each record. If we add the following optional callbacks to type:

@callback set_source(list({record, value})) :: list(value)
@callback group_by_source(list(value)) :: %{primary_key => list(value)}

We can use this to allow loading through types in a single pass.