feat: add "Generate blanket trait impl" assist

[A4-Tacks]

Close #19768

This is very convenient for extending traits such as Iterator

---

Assist: generate_blanket_trait_impl

Generate blanket trait implementation.

trait $0Foo<T: Send>: ToOwned
where
    Self::Owned: Default,
{
    fn foo(&self) -> T;

    fn print_foo(&self) {
        println!("{}", self.foo());
    }
}

->

trait Foo<T: Send>: ToOwned
where
    Self::Owned: Default,
{
    fn foo(&self) -> T;

    fn print_foo(&self) {
        println!("{}", self.foo());
    }
}

$0impl<T: Send, This: ToOwned> Foo<T> for This
where
    Self::Owned: Default,
{
    fn foo(&self) -> T {
        todo!()
    }
}

[A4-Tacks]

@Veykril May I ask if these PRs can be merged?

[A4-Tacks]

r? @Veykril

[A4-Tacks]

r? @ShoyuVanilla

[ShoyuVanilla]

I think this overlaps with #19938? Though this adds blanket impls but I think they share lots of codes and we could reduce duplicates

[A4-Tacks]

I think this overlaps with #19938? Though this adds blanket impls but I think they share lots of codes and we could reduce duplicates

I compared the differences between these two and it doesn't seem like there's much shared code

[ShoyuVanilla]

Okay, I'll look into this more but it might take some time as I have other works for now 😅

[A4-Tacks]

@ShoyuVanilla Do you have time to review this PR?

[ShoyuVanilla]

Sorry, not yet (I've been sick these days 😅 ) I'll review this tomorrow.

[A4-Tacks]

Okay, thank you for taking the time to review