AsRef<str> on paths / attributes should be replaced with std::fmt::Display

[TroyKomodo]

As the title says, currently has a few apis where the function looks like this

pub fn field_attribute<P, A>(&mut self, path: P, attribute: A) -> &mut Self
where
    P: AsRef<str>,
    A: AsRef<str>,
{
    self.field_attributes
        .insert(path.as_ref().to_string(), attribute.as_ref().to_string());
    self
}

Internally this is calling to_string(), so we should simply require the Path or Attribute implement std::fmt::Display

I am using quote! to generate rust attributes

prost.field_attribute(
    &field.full_name,
    quote!(#[serde(serialize_with = #serialize_with)]).to_string(),
);

and TokenStream impls Display but not AsStr<str> .

this isnt a major issue since i can just call to_string() on my side, I just wanted to open this issue to see if we can fit this into a maybe #624 release