Fix EventEngine factory method return types: unique_ptr for lifetime management (#25971)

include/grpc/event_engine/event_engine.h

@@ -166,14 +166,13 @@ class EventEngine {
     /// been called.
     virtual absl::Status Bind(const ResolvedAddress& addr) = 0;
     virtual absl::Status Start() = 0;
-    virtual absl::Status Shutdown() = 0;
   };
 
   // TODO(hork): define status codes for the callback
   // TODO(hork): define return status codes
   // TODO(hork): document status arg meanings for on_accept and on_shutdown
   /// Factory method to create a network listener.
-  virtual absl::StatusOr<Listener> CreateListener(
+  virtual absl::StatusOr<std::unique_ptr<Listener>> CreateListener(
       Listener::AcceptCallback on_accept, Callback on_shutdown,
       const ChannelArgs& args,
       SliceAllocatorFactory slice_allocator_factory) = 0;
@@ -236,7 +235,7 @@ class EventEngine {
 
   // TODO(hork): define return status codes
   /// Retrieves an instance of a DNSResolver.
-  virtual absl::StatusOr<DNSResolver> GetDNSResolver() = 0;
+  virtual absl::StatusOr<std::unique_ptr<DNSResolver>> GetDNSResolver() = 0;
 
   /// Intended for future expansion of Task run functionality.
   struct RunOptions {};