Have a clean way to declare and reference packet tables

In D2Net Client.cpp, the size of the packet sizes table (std::size(gServerPacketSizeTable)) is hardcoded (0xB4).

We should probably have a constant for this, or put the table in its own header since it should only be included in D2Net's Client.cpp and Server.cpp.

Originally posted by @Lectem in https://github.com/ThePhrozenKeep/D2MOO/pull/96#discussion_r1083280497