Fix non_snake_case warning on generated identity functions (#891)

The `Message` derive macro currently generates `Debug` implementations
that contain code that violates the `non_snake_case` linter rule[^1].

This commit changes the macro expansion to add an `#[allow(...)]`
attribute for this rule.

[^1]: https://doc.rust-lang.org/stable/nightly-rustc/rustc_lint/nonstandard_style/static.NON_SNAKE_CASE.html

Co-authored-by: Lucio Franco <[email protected]>

prost-derive/src/field/scalar.rs

@@ -228,6 +228,7 @@ impl Field {
             }
         } else {
             quote! {
+                #[allow(non_snake_case)]
                 fn #wrap_name<T>(v: T) -> T { v }
             }
         }