Add vanguards info to the circuits in hspool::pool::Pool

Today the pool contains a Vec of client circuits:

/// The collection of circuits themselves, in no particular order.
circuits: Vec<Arc<ClientCirc>>,

To support vanguards, we need to know for each circuit if it's STUB or STUB+, and whether it is a vanguards-full or vanguards-lite circuit. It should also be possible to disable vanguards altogether, so this extra vanguards info should be optional.

This information will probably need to be returned, along with the ClientCirc, from CircMgr::launch_hs_unmanaged. This could be included alongside ClientCirc, or as a field within it (gated behind the hs-common feature), depending on what we decide in #1274 (closed).