Add Steam dedicated server support

[Infiland]

Close #157

Summary

• Adds the Steam GameServer lifecycle for dedicated server builds: initialization, callback dispatch, shutdown, server metadata, heartbeat/login helpers, UDP packet relay helpers, auth tickets, and license/status queries.
• Adds hosted dedicated server Steam Datagram Relay helpers alongside the existing networking sockets wrappers.
• Documents the dedicated server setup path and the GameServer-facing APIs.

Audit follow-up

• steam_update() now dispatches SteamGameServer_RunCallbacks() when the GameServer API is initialized, so server-only builds do not silently skip callbacks if they use the shared update hook.
• steam_gameserver_get_next_outgoing_packet() now reports success only when packet bytes were actually written to the provided buffer.
• steam_net_sockets_recv_messages_on_poll_group() no longer frees a struct after transferring it into the result array, matching the ownership pattern used by the socket-pair helper.
• steam_net_sockets_set_connection_name() now returns false when the networking sockets API is unavailable.

Validation

• git diff --check
• xcodebuild -list -project source/Steamworks_gml/extensions/steamworks/steamworks_macos/Steamworks.xcodeproj
• xcodebuild -project source/Steamworks_gml/extensions/steamworks/steamworks_macos/Steamworks.xcodeproj -scheme Steamworks -configuration Debug CODE_SIGNING_ALLOWED=NO build reaches compilation, then stops because this checkout does not include steam_api.h from the Steamworks SDK.
• Checked Valve Steamworks docs for SteamGameServer_Init, GameServer callback dispatch, and the dedicated server lifecycle.

Runtime note: I did not run a live Steam dedicated server from this environment because the local checkout is missing the Steamworks SDK headers required for a native build.

[jzavala-YYG]

This feature it's in our internal TODO list, eventually we will include server support for Steamworks.

@Infiland Did you tested this code with your own server?

[Infiland]

I audited this again and pushed dbea49c with hardening fixes around callback dispatch, outgoing packet write success, and networking socket result ownership.

I have not run this against a live Steam dedicated server from this environment. The local checkout is missing the Steamworks SDK header steam_api.h, so the no-signing Xcode build reaches compilation and then stops at header resolution. Local validation done here: git diff --check, xcodebuild -list, the blocked xcodebuild ... CODE_SIGNING_ALLOWED=NO build attempt, and a cross-check against Valve SteamGameServer lifecycle/callback docs.