Related to #2341

Motivation

Currently, tonic-build generates service traits without any ability to attach additional attributes or proc-macros.

This creates friction when integrating generated traits with tools and frameworks that rely on such attributes.
For example, when using mockall for testing, one cannot simply write:

#[mockall::automock]
pub trait MyService { ... }

because the trait is auto-generated and cannot be easily modified.
As a result, mocking requires duplicating the entire trait or writing manual boilerplate, which is error-prone and difficult to maintain.

Solution

This PR adds a new trait_attribute configuration method to tonic-build, allowing custom attributes (including proc-macros) to be injected into generated service traits.

Example usage:

tonic_build::configure()
    .trait_attribute("myservice.MyService", "#[mockall::automock]")
    .compile_protos(&["proto/myservice.proto"], &["proto"])?;

This will place #[mockall::automock] directly above the generated MyService trait, avoiding manual trait duplication for testing and enabling other use cases (e.g., tracing or custom derives).

I could not find any existing tests in tonic-build that cover this code path, so I have not added automated tests yet.
If the maintainers can point me to where such tests should live, I can add them.