Allow ability to specify serialization key for fields instead of always defaulting to field name

[antoinesaliba]

Goal: Be able to build a struct from another struct even if the field names are different

Context: Serializing a struct currently always makes the keys the name of the fields, this makes this flow not work if you want to build one struct from another:

1. struct1 has a field income_type
2. you serialize your instance of struct1 to a hash
3. you deserialize the hash to build struct2 which has a single field called type

step 3 will raise an error Typed::Validations::RequiredFieldError: type is required. because the hash doesn't have a key for type, but it has a key for `income_type.

Ideally, it'd be great to specify a target for each field. If target is specified, that's what is used for the key name instead of the field name.