Make String::copy public

I have a small feature request (and I am happy to provide the PR ), but why isn't .copy public? There is a use-case here which makes a lot of sense -- specifically when you are given a pointer to a char* that is not null-terminated (and instead are provided a length).

This is exactly how the PubSubClient MQTT library works. The topic callback provides you a byte *payload and length, but because it's byte * it's not null-terminated.

Currently you have to do this (or something like it) to get a String object:

```
void mqttCallback(char *topic, byte *payload, unsigned int length) {
String action;
char *payloadStr;

payloadStr = (char *)malloc(length + 1);
memcpy(payloadStr, payload, length);
payloadStr[length] = 0x0;
action = String(payloadStr);
free(payloadStr);
}
```

When what would make a lot more sense (since String doesn't provide a constructor for this) would be:

```
void mqttCallback(char *topic, byte *payload, unsigned int length) {
String action;
action.copy((char *)payload, length);
}
```

Inizia dall’API Arduino String e ispeziona l’interfaccia e l’implementazione esistenti di String::copy. Verifica come gestisce un puntatore char e una lunghezza, quindi conferma che un payload PubSubClient non terminato da null possa essere copiato senza un’allocazione temporanea e che il comportamento esistente di String rimanga invariato.

Scritto dal modello di indicizzazione a partire dal testo della issue.