Add String(char *, unsigned) constructor

From upstream ArduinoCore-API commit 84314888a8a73b643469c5609beed6d21d1c037a

Author: rcombs

cores/esp32/WString.cpp

@@ -35,6 +35,12 @@ String::String(const char *cstr) {
         copy(cstr, strlen(cstr));
 }
 
+String::String(const char *cstr, unsigned int length) {
+    init();
+    if (cstr)
+        copy(cstr, length);
+}
+
 String::String(const String &value) {
     init();
     *this = value;

cores/esp32/WString.h

@@ -55,6 +55,7 @@ class String {
         // fails, the string will be marked as invalid (i.e. "if (s)" will
         // be false).
         String(const char *cstr = "");
+        String(const char *cstr, unsigned int length);
         String(const String &str);
         String(const __FlashStringHelper *str);
 #ifdef __GXX_EXPERIMENTAL_CXX0X__