ot::Spinel::Decoder::ReadIp6Address() method

Reference to IPv6 address variable to output the value (as `spinel_ipv6addr_t`). On success, the address is copied into the output variable.

Encodes and writes an IPv6 address to current input frame. Before using this method `BeginFrame()` must be called to start and prepare a new input frame. Otherwise, this method does nothing and returns error status `OT_ERROR_INVALID_STATE`. If no buffer space is available, this method will discard and clear the current input frame and return the error status `OT_ERROR_NO_BUFS`.