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/msg_framing.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr/kernel.h>
+
+enum studio_framing_state {
+    FRAMING_STATE_IDLE,
+    FRAMING_STATE_AWAITING_DATA,
+    FRAMING_STATE_ESCAPED,
+    FRAMING_STATE_ERR,
+    FRAMING_STATE_EOF,
+};
+
+#define FRAMING_SOF 0xAB
+#define FRAMING_ESC 0xAC
+#define FRAMING_EOF 0xAD
+
+/**
+ * @brief Process an incoming byte from a frame. Will possibly update the framing state depending on
+ * what data is received.
+ * @retval true if data is a non-framing byte, and is real data to be handled by the upper level
+ * logic.
+ * @retval false if data is a framing byte, and should be ignored. Also indicates the framing state
+ * has been updated.
+ */
+bool studio_framing_process_byte(enum studio_framing_state *frame_state, uint8_t data);