Allow clippy::trivially_copy_pass_by_ref

[Kestrer]

async-trait copies the method definitions as non-trait methods, which can result in the clippy::trivially_copy_pass_by_ref warning being incorrect fired. Example:

#[async_trait]
trait Foo {
    fn foo(&self);
}

#[async_trait]
// ^ clippy::trivially_copy_pass_by_ref warning since the internal `foo` isn't part of the trait
impl Foo for () {
    fn foo(&self) {}
}