[core] fix compilation with clang-16 and libc++/libstdc++ (#33854)

```
[  7%] Building CXX object CMakeFiles/grpc_unsecure.dir/src/core/ext/filters/channel_idle/idle_filter_state.cc.o
In file included from /home/dpronin/.conan2/p/t/grpc7af12e0c3f76b/b/src/src/core/ext/filters/census/grpc_context.cc:26:
In file included from /home/dpronin/.conan2/p/t/grpc7af12e0c3f76b/b/src/src/core/lib/debug/trace.h:34:
In file included from /home/dpronin/.conan2/p/t/grpc7af12e0c3f76b/b/src/src/core/lib/gprpp/global_config.h:91:
In file included from /home/dpronin/.conan2/p/t/grpc7af12e0c3f76b/b/src/src/core/lib/gprpp/global_config_env.h:27:
In file included from /home/dpronin/.conan2/p/t/grpc7af12e0c3f76b/b/src/src/core/lib/gprpp/memory.h:24:
In file included from /usr/include/c++/v1/memory:898:
In file included from /usr/include/c++/v1/__memory/shared_ptr.h:31:
/usr/include/c++/v1/__memory/unique_ptr.h:63:19: error: invalid application of 'sizeof' to an incomplete type 'grpc_server_config_fetcher'
    static_assert(sizeof(_Tp) >= 0, "cannot delete an incomplete type");
                  ^~~~~~~~~~~
/usr/include/c++/v1/__memory/unique_ptr.h:297:7: note: in instantiation of member function 'std::default_delete<grpc_server_config_fetcher>::operator()' requested here
      __ptr_.second()(__tmp);
      ^
/usr/include/c++/v1/__memory/unique_ptr.h:231:5: note: in instantiation of member function 'std::unique_ptr<grpc_server_config_fetcher>::reset' requested here
    reset(__u.release());
    ^
/home/dpronin/.conan2/p/t/grpc7af12e0c3f76b/b/src/src/core/lib/surface/server.h:145:21: note: in instantiation of member function 'std::unique_ptr<grpc_server_config_fetcher>::operator=' requested here
    config_fetcher_ = std::move(config_fetcher);
                    ^
/home/dpronin/.conan2/p/t/grpc7af12e0c3f76b/b/src/include/grpc/grpc.h:458:16: note: forward declaration of 'grpc_server_config_fetcher'
typedef struct grpc_server_config_fetcher grpc_server_config_fetcher;
               ^
[  7%] Built target grpc_python_plugin
```

src/core/lib/surface/server.h: Server::set_config_fetcher() function
must be defined below the definition of the type
'struct grpc_server_config_fetcher' because std::unique_ptr must know
how type looks like at the place where the smart pointer calls a deleter




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---------

Signed-off-by: Denis Pronin <dannftk@yandex.ru>
Co-authored-by: Yash Tibrewal <yashkt@google.com>

src/core/lib/surface/server.h

@@ -62,8 +62,6 @@
 #include "src/core/lib/transport/transport.h"
 #include "src/core/lib/transport/transport_fwd.h"
 
-struct grpc_server_config_fetcher;
-
 namespace grpc_core {
 
 extern TraceFlag grpc_server_channel_trace;
@@ -138,9 +136,7 @@ class Server : public InternallyRefCounted<Server>,
   }
 
   void set_config_fetcher(
-      std::unique_ptr<grpc_server_config_fetcher> config_fetcher) {
-    config_fetcher_ = std::move(config_fetcher);
-  }
+      std::unique_ptr<grpc_server_config_fetcher> config_fetcher);
 
   bool HasOpenConnections() ABSL_LOCKS_EXCLUDED(mu_global_);
 
@@ -510,4 +506,13 @@ struct grpc_server_config_fetcher {
   virtual grpc_pollset_set* interested_parties() = 0;
 };
 
+namespace grpc_core {
+
+inline void Server::set_config_fetcher(
+    std::unique_ptr<grpc_server_config_fetcher> config_fetcher) {
+  config_fetcher_ = std::move(config_fetcher);
+}
+
+}  // namespace grpc_core
+
 #endif  // GRPC_SRC_CORE_LIB_SURFACE_SERVER_H