Proposal: Support custom string, binary, message serialization in prost-deriv

[bickfordb]

It would be convenient to reuse existing structs (like those defined for SeaORM models) to be used by Prost (and subsequently Tonic) for Grpc web apps.

For example, given the following protocol buffer code, it would be convenient to have the subsequent Rust code have a 1-1 mapping.

Without this functionality or similar, one currently has to define/generate intermediate structs to marshal between gRPC/ProtoBuf and ORM structs even though they represent exactly the same information.

Example Protocol Buffer

message Timestamp {
   int64 seconds = 1;
   int32 nanos = 2; 
}

message Date {
    int32 year = 1;
    uint32 month = 2;
    uint32 day = 3;
}

message Account {
  bytes id = 1; // UUID encoded as 4 bytes
  string first_name = 2; // like "Joe"
  string last_name = 3; // like "Biden"
  Timestamp created_at = 4; // account registration timestamp encode as message
  Date birthday = 5; // like Nov 20, 1942 encoded as a message
  string favorite_movie_id = 6; // some other uuid encoded as a string like "7033d448-c14e-4967-af0c-9b3826d6edb8"
  string some_other_datetime = 7; // datetime encoded as a iso8601 string like "2022-09-27 18:00:00.000."
}

Example Rust code

use uuid::Uuid;
use chrono::{NaiveDate, DateTime};

#derive(Message, ...)
struct Account { 
      #[prost(UUIDBytesCodec, tag=1)]
      id: Uuid,
      first_name: String,
      last_name: String,
      #[prost(DateTimeMessageCodec, tag=4)]
      created_at: DateTime,
      #[prost(DateMessageCodec, tag=5)]
      birthday: NaiveDate,
      #[prost(UUIDStringCodec, tag=6)]
      favorite_movie_id: Uuid,
      #[prost(ISO8601DateTime, tag=7)]
      some_other_datetime: DateTime
}