feat: add allow_unused_value bindgen override for fluent APIs

[ivov]

Adds an allow_unused_value bindgen override and applies it to the web routing API in beego so route registrations no longer require let _ =.

import "go:github.com/beego/beego/v2/server/web"
import ctx "go:github.com/beego/beego/v2/server/web/context"

fn main() {
  web.Get("/", |c: Ref<ctx.Context>| {
    c.WriteString("Hello, world!")
  })
  web.Run(":8080")
}

Before this change, every web.Get and sibling registration (Post, Put, Delete, Router, InsertFilter, ...) returned Ref<HttpServer> for fluent chaining, so each call tripped lint.unused_value and forced a let _ = prefix.

[github-actions]

Released in lisette-v0.2.2