Use vanguards path selection in CircMgr::launch_hs_unmanaged.

The path selection will be handled by VanguardHsPathBuilder (a new type similar to, or based on, the existing ExitPathBuilder). See #1279 (closed)

CircMgr::launch_hs_unmanaged will need to take an extra argument specifying whether to use full or lite vanguards when building the circuits.

CircMgr::launch_hs_unmanaged will only create STUB circuits. If full vanguards are enabled, the circuits will be extended by one hop as needed (outside of launch_hs_unmanaged), whenever a STUB+ circuit is required (based on the HsCircKind).

Alternatively, CircMgr::launch_hs_unmanaged could take an argument that specifies whether the circuit to launch is STUB or STUB+. We will likely also need a corresponding TargetCircUsage::HsCircBaseWithVanguards (full|lite) for it. Then, based on the TargetCircUsage, TargetCircUsage::build_path can dispatch to VanguardHsPathBuilder::pick_path (as opposed to the ExitPathBuilder::pick_path currently used for `TargetCircUsage::HsCircBase).

Since launch_hs_circuits_as_needed preemptively populates the HsCircPool with circuits, it will need to be modified too (to launch both STUB and STUB+ circuits).

Prerequisites: #1275 (closed), #1277 (closed), #1279 (closed)