[call-v3] Add some initialization support for the new call architecture (#35301)

Closes #35301

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35301 from ctiller:init-call-review a2022cc485
PiperOrigin-RevId: 590982608

src/core/lib/channel/channel_stack.cc

@@ -28,6 +28,7 @@
 #include <grpc/support/log.h>
 
 #include "src/core/lib/channel/channel_args.h"
+#include "src/core/lib/channel/channel_fwd.h"
 #include "src/core/lib/channel/channel_stack_trace.h"
 #include "src/core/lib/gpr/alloc.h"
 #include "src/core/lib/surface/channel_init.h"
@@ -320,3 +321,19 @@ grpc_channel_stack::MakeServerCallPromise(grpc_core::CallArgs call_args) {
   return ServerNext(grpc_channel_stack_element(this, this->count - 1))(
       std::move(call_args));
 }
+
+void grpc_channel_stack::InitClientCallSpine(
+    grpc_core::CallSpineInterface* call) {
+  for (size_t i = 0; i < count; i++) {
+    auto* elem = grpc_channel_stack_element(this, i);
+    elem->filter->init_call(elem, call);
+  }
+}
+
+void grpc_channel_stack::InitServerCallSpine(
+    grpc_core::CallSpineInterface* call) {
+  for (size_t i = 0; i < count; i++) {
+    auto* elem = grpc_channel_stack_element(this, count - 1 - i);
+    elem->filter->init_call(elem, call);
+  }
+}

src/core/lib/channel/channel_stack.h

@@ -244,6 +244,9 @@ struct grpc_channel_stack {
   MakeClientCallPromise(grpc_core::CallArgs call_args);
   grpc_core::ArenaPromise<grpc_core::ServerMetadataHandle>
   MakeServerCallPromise(grpc_core::CallArgs call_args);
+
+  void InitClientCallSpine(grpc_core::CallSpineInterface* call);
+  void InitServerCallSpine(grpc_core::CallSpineInterface* call);
 };
 
 // A call stack tracks a set of related filters for one call, and guarantees

src/core/lib/transport/transport.h

@@ -491,6 +491,11 @@ class CallHandler {
   const RefCountedPtr<CallSpine> spine_;
 };
 
+struct CallInitiatorAndHandler {
+  CallInitiator initiator;
+  CallHandler handler;
+};
+
 template <typename CallHalf>
 auto OutgoingMessages(CallHalf& h) {
   struct Wrapper {