[prost-build] Add Message and Enum specific attributes

[damelLP]

I have a large-ish api and want to be able to auto-generate derive_builder builders for it similar to here #399.

From the code-generator it seems to be an easy addition of just adding the builder functions.
Example usage looks like this.

        prost_build::Config::new()
        .message_attribute(".", "#[derive(derive_builder::Builder)]")
        .compile_protos(...)

[LucioFranco]

Could you explain why you need a message attribute over https://docs.rs/prost-build/latest/prost_build/struct.Config.html#method.type_attribute using this?

[damelLP]

Hi @LucioFranco, yes, the type attribute is applied to all the different types that get generated, and the derive builder attribute fails to compile on enums.

error: Unsupported shape `enum`
   --> src/gen/api.rs:401:10
    |
401 | #[derive(derive_builder::Builder)]
    |          ^^^^^^^^^^^^^^^^^^^^^^^

As message is mapped to a struct and oneof / enums are mapped to enums this would enable you to generate attributes that are only relevant or correct for a particular type shape.

[LucioFranco]

Got it I think that makes sense.