Add initial TOTP watch face impl

Vendor code from https://github.com/Netthaw/TOTP-MCU to do the
heavy lifting of computing SHA-1 and HMAC and the rest of TOTP

Also implement a date_time to unix timestamp method

movement/watch_faces/complications/totp_face.h

@@ -0,0 +1,19 @@
+#ifndef TOTP_FACE_H_
+#define TOTP_FACE_H_
+
+#include "movement.h"
+
+void totp_face_setup(movement_settings_t *settings, void ** context_ptr);
+void totp_face_activate(movement_settings_t *settings, void *context);
+bool totp_face_loop(movement_event_t event, movement_settings_t *settings, void *context);
+void totp_face_resign(movement_settings_t *settings, void *context);
+
+static const watch_face_t totp_face = {
+    totp_face_setup,
+    totp_face_activate,
+    totp_face_loop,
+    totp_face_resign,
+    NULL
+};
+
+#endif // TOTP_FACE_H_