Function prepare_object 

pub fn prepare_object(
    input: &mut Value,
    ctx: &mut ValidationContext,
    config: ModelConfig,
    specs: &[FieldSpec],
    field: impl FnMut(usize, &mut Value, &mut ValidationContext),
)

Prepare a model object: installs config, checks the input is an object, resolves aliases, reports missing and extra keys, and calls field for each present field with the location set to the key the client sent.