This introduces the `serde_with` dependency and `rust_type.py`, to allow supporting arbitrary types for serialization.
Since fields may have arbitrary types (eg. `HashMap<_, chrono::Duration>`) which need deserialization, it is necessary to
use type-based serialization to avoid implementing (de)serialization for every permutation of types that require special serialization.
However, `serde` does not let you (de)serialize one type as another (eg. `chrono::Duration` as `Wrapper`) - thus necessitating `serde_with`, which does. `rust_type.py` introduces the `RustType` class, which makes it easy to describe the (de)serialization type used by `serde_with`
The serde traits are now directly implemented for FieldMask - this helps address potential serde issues with wrapper types, and simplifies the serde process somewhat.
If using #[serde(with = ...)] with an Option type, serde will expect all marked fields to be present. Adding #[serde(default)] restores expected behaviour - if no Option value is present, None will be used.
