ot::Message::WriteBytesFromMessage() method

Writes bytes read from another or potentially the same message to the message at a given offset. Will not resize the message. The bytes to write (with @p aLength) MUST fit within the existing message buffer (from the given @p aWriteOffset up to the message's length). Can be used to copy bytes within the same message in either direction, i.e., copy forward where `aWriteOffset > aReadOffset` or copy backward where `aWriteOffset < aReadOffset`.