Add type-erased trait and support for shared AnyNodeRef across nodes

[geoffreygarrett]

Replaces element-specific macro implementations with a unified blanket implementation for node references in Leptos 0.7. Enables type-safe node reference handling across element types through a single IntoAnyNodeRef trait implementation.

Adds custom Attribute implementation for AnyNodeRefAttr to enable safe node reference sharing across conditional rendering branches, addressing the discussion about node ref sharing panics (see Discord discussion).

[geoffreygarrett]

To clarify, this implementation:

impl<E: ElementType> NodeRefContainer<E> for AnyNodeRef {
    fn load(self, el: &Element) {
        self.0.set(Some(SendWrapper::new(el.clone())));
    }
}

Replaces the previous use of macros.