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);
}
```

Comienza por la API de Arduino String e inspecciona la interfaz y la implementación existentes de String::copy. Verifica cómo maneja un puntero char y una longitud, y confirma después que una carga útil de PubSubClient no terminada en null se puede copiar sin una asignación temporal y que el comportamiento existente de String se mantiene intacto.

Escrito por el modelo de indexación a partir del texto del issue.