feat: expose HostMemoryOrVec via API

[theHamsta]

Since I needed the same primitive also for RustNN we could expose it in the trtx API. This helper is commonly when you either get owned engine bytes from TensorRT or from a cache.

[theHamsta]

This could also be

enum HostMemoryOrVec<'memory> {
    HostMemory(HostMemory<'memory>),
    Cow(Cow<'memory, [u8]>),
}

impl<'memory> AsRef<[u8]> for HostMemoryOrVec<'memory> {
    fn as_ref(&self) -> &[u8] {
        match self {
            HostMemoryOrVec::HostMemory(host_memory) => host_memory.as_ref(),
            HostMemoryOrVec::Cow(items) => items.as_ref(),
        }
    }
}

impl<'memory> Deref for HostMemoryOrVec<'memory> {
    type Target = [u8];

    fn deref(&self) -> &Self::Target {
        self.as_ref()
    }
}

impl<'buffer> From<HostMemory<'buffer>> for HostMemoryOrVec<'buffer> {
    fn from(value: HostMemory<'buffer>) -> Self {
        HostMemoryOrVec::HostMemory(value)
    }
}

impl<'memory> From<Cow<'memory, [u8]>> for HostMemoryOrVec<'memory> {
    fn from(value: Cow<'memory, [u8]>) -> Self {
        HostMemoryOrVec::Cow(value)
    }
}

so probably better leave to users to implement it themselves 🤷 ?