Obviate typespec_client_core dependency in generated code

[heaths]

As mentioned in Azure/typespec-rust#324, we couldn't get rid of the typespec_client_core dependency in generated crates' Cargo.toml files because of a couple derive procedural macros.

The Model derive macro does already have a solution where we can change the crate referenced used:

#[derive(Model)]
#[typespec(crate = "azure_core")]

But it looks like the SafeDebug derive macro would need the same capability, or find a way to avoid even referencing the typespec_client_core crate. The only reason it does currently is because it actually implements the typespec_client_core::fmt::SafeDebug trait in addition to std::fmt::Debug:

        #[automatically_derived]
        impl #impl_generics ::typespec_client_core::fmt::SafeDebug for #name #ty_generics #where_clause {}

We probably don't even need that trait. I didn't have specific plans for it. The main goal was to safely implement std::fmt::Debug but figured a separate trait could factor into issues regarding sanitization.

See https://github.com/Azure/typespec-rust/pull/324#discussion_r2010755513 for context