feat(endpoints): add preferred endpoint setting

Added a new setting to remember the user's preferred endpoint. When both USB and
BLE are connected, the preferred endpoint will be used.

Added a new behavior to control this setting. It supports commands:

    &end END_USB - Prefer USB output
    &end END_BLE - Prefer BLE output
    &end END_TOG - Toggle between USB and BLE

app/include/zmk/endpoints.h

@@ -9,4 +9,12 @@
 #include <zmk/keys.h>
 #include <zmk/hid.h>
 
+enum zmk_endpoint {
+    ZMK_ENDPOINT_USB,
+    ZMK_ENDPOINT_BLE,
+};
+
+int zmk_endpoints_select(enum zmk_endpoint endpoint);
+int zmk_endpoints_toggle();
+
 int zmk_endpoints_send_report(u8_t usage_report);