feat(stackable-versioned): Add convert_with arg in changed action

[Techassi]

This PR adds a new argument convert_with to the changed() action. It allows users to customize the conversion function used on a field level. This is useful when auto-generated From impl between versions should be kept, but individual fields need custom conversions instead of using the default .into() call.

Example

#[versioned(
    version(name = "v1alpha1"),
    version(name = "v1beta1")
)]
pub struct Foo {
    #[versioned(changed(
        since = "v1beta1",
        from_type = "u8",
        convert_with = "u8_to_u16"
    ))]
    bar: u16,
}

fn u8_to_u16(old: u8) -> u16 {
    old as u16
}

[NickLarsenNZ]

LGTM

[sbernauer]

Very nice quality of life feature!