[core] Make register_component protected, remove runtime checks

Instead of runtime null, duplicate, and capacity checks in
register_component_, make the method unreachable from outside
codegen by removing the public template wrapper and granting
friend access to the codegen-generated ::setup() function.

Since ESPHOME_COMPONENT_COUNT is set to exactly
len(CORE.component_ids) at codegen time, the StaticVector is
always correctly sized and the runtime capacity check cannot
trigger from codegen. External components that bypassed codegen
to call App.register_component() directly will now get a
compile error, forcing them to properly declare their components
in their config schema.

Co-Authored-By: J. Nick Koston <nick@koston.org>

tests/dummy_main.cpp

@@ -16,10 +16,10 @@ void setup() {
   auto *log = new logger::Logger(115200);  // NOLINT
   log->pre_setup();
   log->set_uart_selection(logger::UART_SELECTION_UART0);
-  App.register_component(log);
+  App.register_component_(log);
 
   auto *wifi = new wifi::WiFiComponent();  // NOLINT
-  App.register_component(wifi);
+  App.register_component_(wifi);
   wifi::WiFiAP ap;
   ap.set_ssid("Test SSID");
   ap.set_password("password1");