Should this be added to the String class, so it also supports 64-bit integers?

@Ivan-Perez
The String Class could be extended with two functions (one given here)

String::String(uint64_t value, unsigned char base)
{
init();
char buf[1 + 8 * sizeof(unsigned long)];
ul64toa(value, buf, base);
*this = buf;
}

The AVR stdlib does not have a ul64toa() so you need something like this

// straightforward implementation
void ui64toa(uint64_t v, char * buf, uint8_t base)
{
 int idx = 0;
 uint64_t w = 0;
 while (v > 0)
 {
 w = v / base;
 buf[idx++] = (v - w * base) + '0';
 v = w;
 }
 buf[idx] = 0;
 // reverse char array
 for (int i = 0, j = idx - 1; i < idx / 2; i++, j--)
 {
 char c = buf[i];
 buf[i] = buf[j];
 buf[j] = c;
 }
}

A similar function for a signed version void i64toa()

This PR was merged in arduino/ArduinoCore-API@3c3b27e , closing here