Attribute like `from` for `Box`ing large error variants

Some recent clippy update warns for large enum error variants, and recommends boxing them. This is probably reasonable. It would be convenient if there was an annotation like `from` which would create the box itself rather than having to manually implement an increasingly common use case.

Looks like `thiserror_ext` has a related utility already: https://docs.rs/thiserror-ext/latest/thiserror_ext/derive.Box.html but could probably be cleaner. Something like

```rust
use thiserror::Error;

#[derive(Error, Debug)]
pub enum MyError {
Small(#[from] small::Error),
Big(#[from(autobox)] Box)
}
```

which would generate something like

```rust
use thiserror::Error;

#[derive(Error, Debug)]
pub enum MyError {
Small(small::Error),
Big(Box)
}

impl From for MyError {
fn from(value: small::Error) -> Self {
Self::Small(value)
}
}

impl From for MyError {
fn from(value: big::Error) -> Self {
Self::Big(Box::new(value))
}
}
```

Maybe with an `impl From> for MyError` too, for completeness.