Config option to use proto `enum`s as Rust `enum`s

[tkr-sh]

By default, protos enums are converted to Rust i32. Which can be annoying / not convenient.

I created a new Config field - proto_enum_to_rust_enum - that, when activated, converts proto's enums into Rust's enum instead of i32, The data transferred over the wire is exactly the same as i32. (See: prost-derive/src/field/enum_typed.rs)

---

Not sure where to put the test in prost-build/src/code_generator.rs?

[tkr-sh]

This might be a partial fix to https://github.com/tokio-rs/prost/issues/276 btw

[Ten0]

Related PR: #1079

[tkr-sh]

Yes! But this one is really easy to implement since it keeps the same declaration of enums, it's just that the types used in messages (in the Rust representation) are enums. I think that it could be a nice addition, even to #1079 in case you don't want to handle open enums. Also, would be really easy to add support for Result<_, i32> https://github.com/tokio-rs/prost/pull/1079#issuecomment-2197525664