Customized generation: ordering customized code at the top of message

[vaikzs]

When registering for trait protobuf_codegen::CustomizeCallback for messages. Derive macros with implemented by structs converted from protobuf messages are placed at first line.

This is probably a design decision to consider only custom derive macros to be added before struct. However I have a scenario where custom implementation for struct has to be added.

Example scenario

message Animal {
  string dog = 1; 
}

generated struct

Struct Animal {}

Lets say -- assume following code block has to be placed before struct Animal

impl Animal {
    pub fn  dog() { 
       todo!()   
    }
}

then current protobuf-codegen when writing derives for each struct generates the following:

#[derive(PartialEq, Clone, Default, Debug)]
impl Animal {
    pub fn  dog() { 
       todo!()   
    }
}

struct Animal {}