Function for generating Strings
Returns a random base64 String
base64(integer()) :: String.t()
iex> Faker.String.base64() "1tmLiMhm" iex> Faker.String.base64() "29Tee6SN" iex> Faker.String.base64(5) "Kfp7+" iex> Faker.String.base64(100) "KLJyZ7xbfJZPMy3J7dAsyfOB3vnZIqFGv4VQil8D/xh1C/Nj9K7xJk47zJtcKsy5mjpJk61Wt3jcJu3bfgwuScTmOOYt4ykzvDUl"