Support for STMT_ATTR_CB_RESULT_EX callback

Connector/Python currently registers field_fetch_callback via STMT_ATTR_CB_RESULT. The callback receives only a pointer into the raw row (unsigned char **row) and no end-of-buffer pointer, so it cannot verify that the data it parses is within bounds. Length-encoded values, fixed-size reads and temporal values are all trusted as sent by the server. With a truncated or malformed row, the callback can read past the end of the buffer.

Once C/C provides STMT_ATTR_CB_RESULT_EX (callback with end pointer and an error return), Connector/Python should use it with full bounds checking. If the library does not support the new attribute, Connector/Python must not use the old callback and should fetch results through the standard bind-buffer path.