Allow builder_struct_attr to set attributes before #[derive(...)]

[mmarkmos]

I use this lib to have "duplicates" of my structs where each field is Optional for serde. I want use skip_serializing_none macro so that I don't have to repeat #[builder_field_attr(serde(skip_serializing_if = "Option::is_none"))] on every field. But this attribute has to be set before the derive to work.

[TedDriggs]

According to this comment, those attrs can't come before the derives without causing a compiler warning. It sounds like this would require another tweak to #[builder_struct_attr(...)] to work; probably another new attribute name.