[ruby] Fix compilation errors on clang 16

As announced in
https://discourse.llvm.org/t/clang-16-notice-of-potentially-breaking-changes/65562,
clang 16 defaults `-Wincompatible-function-pointer-types` to be
on. This causes a number of hard errors due to implicit conversions
between `void *` and `void`, such as:

```
../../../../src/ruby/ext/grpc/rb_channel.c:770:47: error: incompatible function pointer types passing 'VALUE (VALUE)' (aka 'unsigned long (unsigned long)') to parameter of type 'VALUE (*)(void *)' (aka 'unsigned long (*)(void *)') [-Wincompatible-function-pointer-types]
  g_channel_polling_thread = rb_thread_create(run_poll_channels_loop, NULL);
                                              ^~~~~~~~~~~~~~~~~~~~~~
/root/.rbenv/versions/3.1.4/include/ruby-3.1.0/ruby/internal/intern/thread.h:190:32: note: passing argument to parameter 'f' here
VALUE rb_thread_create(VALUE (*f)(void *g), void *g);
                               ^
../../../../src/ruby/ext/grpc/rb_channel.c:780:41: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
void grpc_rb_channel_polling_thread_stop() {
                                        ^
                                         void
../../../../src/ruby/ext/grpc/rb_channel.c:786:30: error: incompatible function pointer types passing 'void (void *)' to parameter of type 'void *(*)(void *)' [-Wincompatible-function-pointer-types]
  rb_thread_call_without_gvl(run_poll_channels_loop_unblocking_func, NULL, NULL,
```

This commit fixes these pointer types using wrapper functions where
necessary.

This issue was also raised on the FreeBSD port of the grpc gem:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271540

src/ruby/ext/grpc/rb_channel.c

@@ -117,6 +117,7 @@ static bg_watched_channel* bg_watched_channel_list_create_and_add(
     grpc_channel* channel);
 static void bg_watched_channel_list_free_and_remove(bg_watched_channel* bg);
 static void run_poll_channels_loop_unblocking_func(void* arg);
+static void* run_poll_channels_loop_unblocking_func_wrapper(void* arg);
 
 // Needs to be called under global_connection_polling_mu
 static void grpc_rb_channel_watch_connection_state_op_complete(
@@ -689,8 +690,12 @@ static void* run_poll_channels_loop_no_gil(void* arg) {
   return NULL;
 }
 
-// Notify the channel polling loop to cleanup and shutdown.
 static void run_poll_channels_loop_unblocking_func(void* arg) {
+  run_poll_channels_loop_unblocking_func_wrapper(arg);
+}
+
+// Notify the channel polling loop to cleanup and shutdown.
+static void* run_poll_channels_loop_unblocking_func_wrapper(void* arg) {
   bg_watched_channel* bg = NULL;
   (void)arg;
 
@@ -701,7 +706,7 @@ static void run_poll_channels_loop_unblocking_func(void* arg) {
   // early out after first time through
   if (g_abort_channel_polling) {
     gpr_mu_unlock(&global_connection_polling_mu);
-    return;
+    return NULL;
   }
   g_abort_channel_polling = 1;
 
@@ -723,10 +728,11 @@ static void run_poll_channels_loop_unblocking_func(void* arg) {
   gpr_log(GPR_DEBUG,
           "GRPC_RUBY: run_poll_channels_loop_unblocking_func - end aborting "
           "connection polling");
+  return NULL;
 }
 
 // Poll channel connectivity states in background thread without the GIL.
-static VALUE run_poll_channels_loop(VALUE arg) {
+static VALUE run_poll_channels_loop(void* arg) {
   (void)arg;
   gpr_log(
       GPR_DEBUG,
@@ -783,8 +789,8 @@ void grpc_rb_channel_polling_thread_stop() {
             "GRPC_RUBY: channel polling thread stop: thread was not started");
     return;
   }
-  rb_thread_call_without_gvl(run_poll_channels_loop_unblocking_func, NULL, NULL,
-                             NULL);
+  rb_thread_call_without_gvl(run_poll_channels_loop_unblocking_func_wrapper,
+                             NULL, NULL, NULL);
   rb_funcall(g_channel_polling_thread, rb_intern("join"), 0);
   // state associated with the channel polling thread is destroyed, reset so
   // we can start again later

src/ruby/ext/grpc/rb_event_thread.c

@@ -106,17 +106,22 @@ static void* grpc_rb_wait_for_event_no_gil(void* param) {
   return NULL;
 }
 
-static void grpc_rb_event_unblocking_func(void* arg) {
+static void* grpc_rb_event_unblocking_func_wrapper(void* arg) {
   (void)arg;
   gpr_mu_lock(&event_queue.mu);
   event_queue.abort = true;
   gpr_cv_signal(&event_queue.cv);
   gpr_mu_unlock(&event_queue.mu);
+  return NULL;
+}
+
+static void grpc_rb_event_unblocking_func(void* arg) {
+  grpc_rb_event_unblocking_func_wrapper(arg);
 }
 
 /* This is the implementation of the thread that handles auth metadata plugin
  * events */
-static VALUE grpc_rb_event_thread(VALUE arg) {
+static VALUE grpc_rb_event_thread(void* arg) {
   grpc_rb_event* event;
   (void)arg;
   while (true) {
@@ -155,7 +160,8 @@ void grpc_rb_event_queue_thread_stop() {
             "GRPC_RUBY: call credentials thread stop: thread not running");
     return;
   }
-  rb_thread_call_without_gvl(grpc_rb_event_unblocking_func, NULL, NULL, NULL);
+  rb_thread_call_without_gvl(grpc_rb_event_unblocking_func_wrapper, NULL, NULL,
+                             NULL);
   rb_funcall(g_event_thread, rb_intern("join"), 0);
   g_event_thread = Qnil;
 }