Update unit.pyi with latest systemd D-Bus interfaces

Add new properties and methods from systemd's org.freedesktop.systemd1.Unit
and org.freedesktop.systemd1.Service interfaces while preserving backward
compatibility with existing types.

Unit interface additions:
- New properties: AccessSELinuxContext, ActivationDetails, CanFreeze,
  CanLiveMount, DebugInvocation, FreezerState, Markers, OnFailureOf,
  OnSuccess, OnSuccessJobMode, OnSuccessOf, PropagatesStopTo, SliceOf,
  StopPropagatedFrom, SurviveFinalKillSignal, UpheldBy, Upholds, WantsMountsFor
- New methods: Freeze, Thaw, KillSubgroup, QueueSignal

Service interface additions:
- BPF delegation properties
- Credential management (LoadCredential, SetCredential, ImportCredential)
- Memory management (MemoryPeak, MemoryZSwap*, ManagedOOM*)
- Namespace/container settings (PrivateIPC, PrivatePIDs, ProtectClock, etc.)
- Directory quota properties
- New methods: BindMount, DumpFileDescriptorStore, MountImage, RemoveSubgroup

Author: aleivag