Multiple FromParts in handlers (#1999)

* Multiple FromParts in handlers

Handlers can have up to 8 extensions:
```
pub async fn handler(
    input: input::Input,
    ext0: Extension<...>,
    ext1: Extension<...>,
    ext2: Extension<...>,
    ext3: Extension<...>,
    ext4: Extension<...>,
    ext5: Extension<...>,
    ext6: Extension<...>,
    ext7: Extension<...>,
) -> Result<output::Output, error::Error> { ... }
```

Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>
Co-authored-by: Harry Barber <hlbarber@amazon.co.uk>