feat: Add keyboard physical layout system.

* Add bindings to allow creating multiple physical layouts that specify
  their key's physical attributes, and the matching matrix transform
  and dependant kscan to use.
* Synthesize a basic physical layout if none specified, for backwards
  compatibility.
* Update matrix transform API to explicitly pass in the selected transform
  to the API instead of using a fixed chosen transform.
* Move kscan subscription and handling into the physical layout code, so
  that selecting a different physical layout at runtime can also use the
  correct kscan instance.
* Add `physical_layouts.dtsi` file to include so you can use the
  pre-configured `&key_physical_attrs` for adding you layout keys.

app/include/zmk/physical_layouts.h

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr/kernel.h>
+#include <zmk/matrix_transform.h>
+
+struct zmk_key_physical_attrs {
+    int16_t width;
+    int16_t height;
+    int16_t x;
+    int16_t y;
+    int16_t rx;
+    int16_t ry;
+    int16_t r;
+};
+
+struct zmk_physical_layout {
+    const char *display_name;
+
+    zmk_matrix_transform_t matrix_transform;
+    const struct device *kscan;
+
+    const struct zmk_key_physical_attrs *keys;
+    size_t keys_len;
+};
+
+#define ZMK_PHYS_LAYOUTS_FOREACH(_ref) STRUCT_SECTION_FOREACH(zmk_physical_layout, _ref)
+
+size_t zmk_physical_layouts_get_list(struct zmk_physical_layout const *const **phys_layouts);
+
+int zmk_physical_layouts_select(uint8_t index);
+int zmk_physical_layouts_get_selected(void);
+
+int zmk_physical_layouts_check_unsaved_selection(void);
+int zmk_physical_layouts_save_selected(void);
+int zmk_physical_layouts_revert_selected(void);
+
+int zmk_physical_layouts_get_position_map(uint8_t source, uint8_t dest, uint32_t *map);