Structs impling Default should have default Python constructors

[adamchalmers]

Currently pyo3 bindings require you to explicitly annotate any Rust "constructor" as a Python constructor. This means that if your struct relies on its default() method from the Default trait you cannot expose the default() method directly. Instead you have to do this:

#[pymethods]
impl MyStruct {
    #[new]
    fn new() -> Self {
        Self::default()
    }
}

In my opinion, exposing this default() method is a common enough problem that this boilerplate is a little unnecessary. What if instead we could do

#[derive(Default)]
#[pyclass(default)]
struct MyStruct;

Here the pyclass(default) would create a zero-argument constructor method for the Python type, and its implementation would call MyStruct::default.

[Rafa-Gu98]

This is a constructive discussion :)
To move this forward, I'd suggest:

1. Attribute Precedence
   If both manual #[new] and #[pyclass(default)] exist, manual implementation should take priority to avoid conflicts

2. Generic Support
   Should work seamlessly with T: Default generics (e.g., MyStruct<T> automatically gets __init__(self))

3. Compile-time Safety
   Clear error if #[pyclass(default)] is applied to non-Default types

[davidhewitt]

Lots of overlap with #5416 / #5421, it would be awesome to figure out a design which solves both elegantly.