Add #[pyo3(default)] option for FromPyObject derives

[davidhewitt]

Continuing from #3605

The request is to add a #[pyo3(default)] attribute to allow missing values to be filled in, similar to serde. Note that serde allows Option<T> fields to be missing and fills them with None by default without requiring the attribute; in the linked discussion we agreed that was too implicit for us here.

    use pyo3::FromPyObject;
    #[derive(FromPyObject, Clone)]
    #[pyo3(from_item_all)]
    struct Test {
        pub a: String,
        #[pyo3(default)]    // <-- this would be the new attribute
        pub b: Option<String>,
    }

    #[test]
    fn test_option() -> Result<()> {
        Python::with_gil(|py| {
            let result: Test =
                py.eval("{'a': 'test'}", None, None).wrap_err("eval failed")?.extract()?;
            assert!(result.a == "test");
            assert!(result.b.is_none());
            Ok(())
        })
    }