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/CMakeLists.txt

@@ -105,4 +105,27 @@ target_sources(app PRIVATE src/main.c)
 add_subdirectory(src/display/)
 add_subdirectory_ifdef(CONFIG_SETTINGS src/settings/)
 
+if (CONFIG_ZMK_STUDIO_RPC)
+  # For some reason this is failing if run from a different sub-file.
+  list(APPEND CMAKE_MODULE_PATH ${ZEPHYR_BASE}/modules/nanopb)
+
+  include(nanopb)
+
+    # Turn off the default nanopb behavior
+  set(NANOPB_GENERATE_CPP_STANDALONE OFF)
+
+  nanopb_generate_cpp(proto_srcs proto_hdrs RELPATH ${ZEPHYR_ZMK_STUDIO_MESSAGES_MODULE_DIR}
+                        ${ZEPHYR_ZMK_STUDIO_MESSAGES_MODULE_DIR}/proto/zmk/studio.proto
+                        ${ZEPHYR_ZMK_STUDIO_MESSAGES_MODULE_DIR}/proto/zmk/meta.proto
+                        ${ZEPHYR_ZMK_STUDIO_MESSAGES_MODULE_DIR}/proto/zmk/core.proto
+                        ${ZEPHYR_ZMK_STUDIO_MESSAGES_MODULE_DIR}/proto/zmk/behaviors.proto
+                        ${ZEPHYR_ZMK_STUDIO_MESSAGES_MODULE_DIR}/proto/zmk/keymap.proto
+  )
+
+  target_include_directories(app PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
+  target_sources(app PRIVATE ${proto_srcs} ${proto_hdrs})
+
+  add_subdirectory(src/studio)
+endif()
+
 zephyr_cc_option(-Wfatal-errors)