Add String(const uint8_t *, unsigned int) constructor

From upstream ArduinoCore-API commit 0d83f1afc3367037dbde5323c2abd0ae1bd2c583

Author: rcombs

Diff for: cores/esp32/WString.h

@@ -56,6 +56,9 @@ class String {
         // be false).
         String(const char *cstr = "");
         String(const char *cstr, unsigned int length);
+#ifdef __GXX_EXPERIMENTAL_CXX0X__
+        String(const uint8_t *cstr, unsigned int length) : String((const char*)cstr, length) {}
+#endif
         String(const String &str);
         String(const __FlashStringHelper *str);
 #ifdef __GXX_EXPERIMENTAL_CXX0X__