[core] Introduce API to collect placed symbols data

The following methods are added to the `Renderer` class:
- `collectPlacedSymbolData()`
  enables or disables collecting of the placed symbols data

- `getPlacedSymbolsData()`
  if collecting of the placed symbols data is enabled, returns the
  reference to the `PlacedSymbolData` vector holding the collected data.

src/mbgl/text/placement.cpp

@@ -1142,6 +1142,11 @@ bool Placement::hasTransitions(TimePoint now) const {
            transitionOptions.duration.value_or(util::DEFAULT_TRANSITION_DURATION);
 }
 
+const std::vector<PlacedSymbolData>& Placement::getPlacedSymbolsData() const {
+    const static std::vector<PlacedSymbolData> data;
+    return data;
+}
+
 const CollisionIndex& Placement::getCollisionIndex() const {
     return collisionIndex;
 }