feat(studio): Initial RPC infrastructure and subsystems.

* UART and BLE/GATT transports for a protobuf encoded RPC
  request/response protocol.
* Custom framing protocol is used to frame a give message.
* Requests/responses are divided into major "subsystems" which
  handle requests and create response messages.
* Notification support, including mapping local events to RPC
  notifications by a given subsystem.
* Meta responses for "no response" and "unlock needed".
* Initial basic lock state support in a new core section, and allow specifying
  if a given RPC callback requires unlocked state or not.
* Add behavior subsystem with full metadata support and examples of
  using callback to serialize a repeated field without extra stack space needed.

Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>

app/src/studio/core.c

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zmk/studio/core.h>
+
+ZMK_EVENT_IMPL(zmk_studio_core_lock_state_changed);
+
+static enum zmk_studio_core_lock_state state = IS_ENABLED(CONFIG_ZMK_STUDIO_LOCKING)
+                                                   ? ZMK_STUDIO_CORE_LOCK_STATE_LOCKED
+                                                   : ZMK_STUDIO_CORE_LOCK_STATE_UNLOCKED;
+
+enum zmk_studio_core_lock_state zmk_studio_core_get_lock_state(void) { return state; }
+
+static void set_state(enum zmk_studio_core_lock_state new_state) {
+    if (state == new_state) {
+        return;
+    }
+
+    state = new_state;
+
+    raise_zmk_studio_core_lock_state_changed(
+        (struct zmk_studio_core_lock_state_changed){.state = state});
+}
+
+#if CONFIG_ZMK_STUDIO_LOCK_IDLE_TIMEOUT_SEC > 0
+
+static void core_idle_lock_timeout_cb(struct k_work *work) { zmk_studio_core_lock(); }
+
+K_WORK_DELAYABLE_DEFINE(core_idle_lock_timeout, core_idle_lock_timeout_cb);
+
+void zmk_studio_core_reschedule_lock_timeout() {
+    k_work_reschedule(&core_idle_lock_timeout, K_SECONDS(CONFIG_ZMK_STUDIO_LOCK_IDLE_TIMEOUT_SEC));
+}
+
+#else
+
+void zmk_studio_core_reschedule_lock_timeout() {}
+
+#endif
+
+void zmk_studio_core_unlock() {
+    set_state(ZMK_STUDIO_CORE_LOCK_STATE_UNLOCKED);
+
+    zmk_studio_core_reschedule_lock_timeout();
+}
+
+void zmk_studio_core_lock() { set_state(ZMK_STUDIO_CORE_LOCK_STATE_LOCKED); }