More accurate packet sizes

[UkoeHB]

Problem

Currently renet and this fork are configured to have 1300 byte payloads. However, the netcode implementation uses 1200 bytes as the max payload size (plus 17-25 bytes for the header and MAC).

This matches quinn which calls for a 1200 byte MTU. However, quinn adds additional headers. I measured around 70 bytes for clients, and 90 bytes for servers (via Wireshark).

Solution

• Change the default payload size back to 1200 for UDP sockets.
• Allow sockets to specify their own max payload size. This opens the way for mega-sized packets for custom data channels that can support it (e.g. in-memory channels can be configured to have 64kb payloads).
• Set max payload size to 1100 bytes for the WebTransport socket, to account for the additional headers/encryption.