Function select_middle_for_vanguard_circ 

pub(crate) fn select_middle_for_vanguard_circ<'n, R: Rng, T: HasRelayIds + 'n>(
    hops: &[T],
    netdir: &'n NetDir,
    selector: &RelaySelector<'n>,
    rng: &mut R,
) -> Result<Relay<'n>>

Select a middle relay that can be appended to a vanguard circuit.

Used by PathBuilder to build TorPaths of the form

G - L2 - M G - L2 - L3 - M

If full vanguards are enabled, this is also used by HsCircPool, for extending NAIVE circuits to become GUARDED circuits.