Generate serialization methods (ctor + method) for partial types

[ReubenBond]

Generate serialization method and deserialization constructor for partial types, useful for types with readonly/initonly/private fields.

The generated ctor must accept dependencies (codecs) as well as the reader and other parameters via the constructor. Therefore, a fixed signature is not suitable.

If there are no existing constructors, also add a default constructor if it's a reference type.