Expose LevelizeObserver / StaLevelizeObserver + add Sta::setLevelizeObserver

[dsengupta0628]

Three small additions to let downstream consumers (OpenROAD's dbSta) attach
their own LevelizeObserver without:

• replicating StaLevelizeObserver's Search + GraphDelayCalc forwarding
  (silent drift risk if this class gains new responsibilities upstream),
• reaching into the protected levelize_ member,
• including the private search/Levelize.hh header.

Changes

1. New public header include/sta/LevelizeObserver.hh — holds
   LevelizeObserver (abstract) and StaLevelizeObserver (default impl that
   forwards level-change events to Search and GraphDelayCalc). Both classes
   moved from their previous private locations.
2. search/Levelize.hh — drop the inline LevelizeObserver definition;
   include the new public header instead.
3. search/Sta.cc — drop the file-local StaLevelizeObserver class body;
   keep its method definitions (now defining the public class).
4. include/sta/Sta.hh — add public
   void Sta::setLevelizeObserver(LevelizeObserver *).
5. search/Sta.cc — implement Sta::setLevelizeObserver as a one-line
   forward to levelize_->setObserver(observer).

No behavior change. Sta::makeObservers still installs StaLevelizeObserver
exactly as before.

Why

OpenROAD dbSta needs to:

• subclass a LevelizeObserver to invalidate its driver-vertex cache when
  levels change, and
• preserve the existing Search/GraphDelayCalc forwarding that StaLevelizeObserver
  provides.

Today the only way is to:

• add ${OPENSTA_HOME} to its private include path (since search/Levelize.hh
  isn't public),
• reach into Sta::levelize_ directly,
• copy the body of StaLevelizeObserver into a dbSta-local subclass (drift
  risk if upstream ever changes StaLevelizeObserver).