ruby: register grpc_rb_sStatus as a global variable (#36125)

Ref: https://bugs.ruby-lang.org/issues/20311

C global variable that contain references to Ruby object MUST be declared to the Ruby GC to prevent these objects from being collected or moved.

There are a few exceptions to that, such as classes defined using the C APIs such as `rb_define_class` etc.

Up to Ruby 3.4 however, there was a bug that caused classes created from Ruby with the `Struct.new("Name")` API to also be marked as immortal and immovable.

GRPC has been relying on this bug, which I fixed in Ruby 3.4, and now GRPC is crashing when `Struct::Status` is moved around by the GC.

```
-- C level backtrace information -------------------------------------------
ruby(rb_print_backtrace+0x14) [0x5577db219d41] ruby-3.4.0/vm_dump.c:820
ruby(rb_vm_bugreport) ruby-3.4.0/vm_dump.c:1151
ruby(rb_bug_for_fatal_signal+0xfc) [0x5577db3cc60c] ruby-3.4.0/error.c:1066
ruby(sigsegv+0x4d) [0x5577db16358d] ruby-3.4.0/signal.c:926
libc.so.6(0x7f5bacd32520) [0x7f5bacd32520]
ruby(rb_class_superclass+0x32) [0x5577db0a9152] ruby-3.4.0/object.c:2239
ruby(struct_ivar_get+0x2a) [0x5577db194e02] ruby-3.4.0/struct.c:49
ruby(struct_ivar_get) ruby-3.4.0/struct.c:40
ruby(num_members) ruby-3.4.0/struct.c:705
ruby(rb_struct_new+0x56) [0x5577db19a9d6] ruby-3.4.0/struct.c:848
lib/grpc/grpc_c.so(grpc_run_batch_stack_build_result+0xe6) [0x7f5b84bb6b96] /tmp/bundle/ruby/3.4.0+0/bundler/gems/grpc-5ed33ee673e3/src/ruby/ext/grpc/rb_call.c:780
lib/grpc/grpc_c.so(grpc_rb_call_run_batch_try) /tmp/bundle/ruby/3.4.0+0/bundler/gems/grpc-5ed33ee673e3/src/ruby/ext/grpc/rb_call.c:839
ruby(rb_ensure+0x10c) [0x5577db007d3c] ruby-3.4.0/eval.c:1000
/tmp/bundle/ruby/3.4.0+0/bundler/gems/grpc-5ed33ee673e3/src/ruby/lib/grpc/grpc_c.so(grpc_rb_call_run_batch+0xca) [0x7f5b84bb595a] /tmp/bundle/ruby/3.4.0+0/bundler/gems/grpc-5ed33ee673e3/src/ruby/ext/grpc/rb_call.c:893
ruby(vm_call_cfunc_with_frame_+0x117) [0x5577db1f4c77] ruby-3.4.0/vm_insnhelper.c:3524
```

cc @apolcyn @peterzhu2118 @k0kubun

Closes #36125

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36125 from Shopify:fix-ruby-3.4-compat 7a127599c8
PiperOrigin-RevId: 616152904

src/ruby/ext/grpc/rb_grpc.c

@@ -467,6 +467,7 @@ void Init_grpc_c() {
   grpc_rb_mGrpcCore = rb_define_module_under(grpc_rb_mGRPC, "Core");
   grpc_rb_sNewServerRpc = rb_struct_define(
       "NewServerRpc", "method", "host", "deadline", "metadata", "call", NULL);
+  rb_global_variable(&grpc_rb_sStatus);
   grpc_rb_sStatus = rb_const_get(rb_cStruct, rb_intern("Status"));
   sym_code = ID2SYM(rb_intern("code"));
   sym_details = ID2SYM(rb_intern("details"));