Config option to use proto enums as Rust enums

[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)

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

[tkr-sh]

This might be a partial fix to #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> #1079 (comment)