As a developer, I want to extend config for header in rust bindings

[linnefromice]

Is your feature request related to a problem? Please describe.

I want to update in config a header that affects the entire structure generated from candid.

let contents = candid::bindings::rust::compile(&config, &self.type_env, &None);
let mut lines = contents
  .lines()
  .skip(4);
lines.insert(0, "#![allow(dead_code, unused_imports)]".to_string());
lines.insert(1,"use candid::{self, CandidType, Deserialize, Principal, Encode, Decode};".to_string());

Describe the solution you'd like

I want to be able to change the following in header with option

• 'use' Declaration
• 'derive' of 'use' declaration
• (if possible) header comment

I have a few requests for structures as well.

• Whether the type, enum, and struct declarations in candid crate include modules or not
  • candid:Xxx or Xxx
    • Now only Principal is without module and the others contain module names.
    • fa29c70#diff-3c5bdad1fc69db86d5703bd188c015ceff01da3b9c0259b0c8f262629148a959L82-R117