atchops_base64_encode: return null-terminated buffer

[XavierChanth]

atchops_base64_encode requires a pre-allocated buffer, which is an anti-pattern for returning null-terminated strings. We can coerce our own null-terminated string by allocating one byte longer than we pass as the buffer size into this function. The style for this function should change to not require input or output parameters, and the function should determine and allocate the buffer itself.