Actually be able to pass a Footer when opening a file

[delta003]

I'd like to pass an externally stored footer when opening a file

vortex/vortex-file/src/open.rs

Line 35 in 029af03

pub(crate) footer: Option<Footer>,

Although this is possible in the API, in practice, it's not usable. There is no (good) way to obtain a Footer at write time, and even if I work around that (for example, by opening a written file again), I have no way to store the footer.