ot::Ip6::PadOption::InitToPadHeaderWithSize() method

Initializes the Pad Option for padding an IPv6 Extension header with a given current size. The Extension Header Length is in 8-bytes unit, so the total size should be a multiple of 8. This method determines the Pad Option size needed for appending to Extension Header based on it current size @p aHeaderSize so to make it a multiple of 8. This method returns `kErrorAlready` when the @p aHeaderSize is already a multiple of 8 (i.e., no padding is needed).