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.c

@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zephyr/logging/log.h>
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#include "msg_framing.h"
+
+static bool process_byte_err_state(enum studio_framing_state *rpc_framing_state, uint8_t c) {
+    switch (c) {
+    case FRAMING_EOF:
+        *rpc_framing_state = FRAMING_STATE_IDLE;
+        return false;
+    case FRAMING_SOF:
+        *rpc_framing_state = FRAMING_STATE_AWAITING_DATA;
+        return false;
+    default:
+        LOG_WRN("Discarding unexpected data 0x%02x", c);
+        return false;
+    }
+
+    return false;
+}
+
+static bool process_byte_idle_state(enum studio_framing_state *rpc_framing_state, uint8_t c) {
+    switch (c) {
+    case FRAMING_SOF:
+        *rpc_framing_state = FRAMING_STATE_AWAITING_DATA;
+        return false;
+    default:
+        LOG_WRN("Expected SOF, got 0x%02x", c);
+        return false;
+    }
+    return false;
+}
+
+static bool process_byte_awaiting_data_state(enum studio_framing_state *rpc_framing_state,
+                                             uint8_t c) {
+    switch (c) {
+    case FRAMING_SOF:
+        LOG_WRN("Unescaped SOF mid-data");
+        *rpc_framing_state = FRAMING_STATE_ERR;
+        return false;
+    case FRAMING_ESC:
+        *rpc_framing_state = FRAMING_STATE_ESCAPED;
+        return false;
+    case FRAMING_EOF:
+        *rpc_framing_state = FRAMING_STATE_EOF;
+        return false;
+    default:
+        return true;
+    }
+
+    return false;
+}
+
+static bool process_byte_escaped_state(enum studio_framing_state *rpc_framing_state, uint8_t c) {
+    *rpc_framing_state = FRAMING_STATE_AWAITING_DATA;
+    return true;
+}
+
+bool studio_framing_process_byte(enum studio_framing_state *rpc_framing_state, uint8_t c) {
+    switch (*rpc_framing_state) {
+    case FRAMING_STATE_ERR:
+        return process_byte_err_state(rpc_framing_state, c);
+    case FRAMING_STATE_IDLE:
+    case FRAMING_STATE_EOF:
+        return process_byte_idle_state(rpc_framing_state, c);
+    case FRAMING_STATE_AWAITING_DATA:
+        return process_byte_awaiting_data_state(rpc_framing_state, c);
+    case FRAMING_STATE_ESCAPED:
+        return process_byte_escaped_state(rpc_framing_state, c);
+    default:
+        LOG_ERR("Unsupported framing state: %d", *rpc_framing_state);
+        return false;
+    }
+}