impl `.cid()` method for `Vlad`

[DougAnderson444]

So we can get the vlad cid from the foot entry vlad/cid but we need a way to ensure it actually matches the one IN the vlad, the entry should be compared to the vlad.

There's no method for this and the field is only pub(crate)

#[derive(Clone, Default, Eq, Ord, PartialEq, PartialOrd)]
pub struct Vlad {
    /// the random nonce for uniqueness
    pub(crate) nonce: Nonce,
    /// validation function content address
    pub(crate) cid: Cid,
}