[ee] Make default event engine factory return a shared pointer (grpc#38970)

Closes grpc#38970

COPYBARA_INTEGRATE_REVIEW=grpc#38970 from eugeneo:shared-ptr-ee e93dd01
PiperOrigin-RevId: 739301045

Author: eugeneo

src/core/lib/event_engine/default_event_engine_factory.cc

@@ -24,8 +24,8 @@
 
 namespace grpc_event_engine::experimental {
 
-std::unique_ptr<EventEngine> DefaultEventEngineFactory() {
-  return std::make_unique<WindowsEventEngine>();
+std::shared_ptr<EventEngine> DefaultEventEngineFactory() {
+  return std::make_shared<WindowsEventEngine>();
 }
 
 }  // namespace grpc_event_engine::experimental
@@ -34,8 +34,8 @@ std::unique_ptr<EventEngine> DefaultEventEngineFactory() {
 
 namespace grpc_event_engine::experimental {
 
-std::unique_ptr<EventEngine> DefaultEventEngineFactory() {
-  return std::make_unique<CFEventEngine>();
+std::shared_ptr<EventEngine> DefaultEventEngineFactory() {
+  return std::make_shared<CFEventEngine>();
 }
 
 }  // namespace grpc_event_engine::experimental
@@ -44,8 +44,8 @@ std::unique_ptr<EventEngine> DefaultEventEngineFactory() {
 
 namespace grpc_event_engine::experimental {
 
-std::unique_ptr<EventEngine> DefaultEventEngineFactory() {
-  return std::make_unique<PosixEventEngine>();
+std::shared_ptr<EventEngine> DefaultEventEngineFactory() {
+  return std::make_shared<PosixEventEngine>();
 }
 
 }  // namespace grpc_event_engine::experimental

src/core/lib/event_engine/default_event_engine_factory.h

@@ -23,7 +23,7 @@
 namespace grpc_event_engine::experimental {
 
 /// Create an EventEngine using the default factory provided at link time.
-std::unique_ptr<EventEngine> DefaultEventEngineFactory();
+std::shared_ptr<EventEngine> DefaultEventEngineFactory();
 
 }  // namespace grpc_event_engine::experimental