Implement generic incoming UDP packet rate limiter (Issue #17)

[FouL06]

Resolves #17

Implements a generic incoming packet rate limiter for direct UDP connections using TokenBucketRateLimiter, protecting basic UDP connections against packet flooding and DoS attacks.

Overview

• include/steam/steamnetworkingtypes.h / src/steamnetworkingsockets/clientlib/csteamnetworkingsockets.cpp:
  • Added connection-level configuration values:
    • k_ESteamNetworkingConfig_RecvRateLimit_PacketRate: Steady-state rate in packets/sec (default 0 = disabled).
    • k_ESteamNetworkingConfig_RecvRateLimit_PacketBurst: Max burst allowed (default 0 = defaults capacity to RecvRateLimit_PacketRate).
• src/steamnetworkingsockets/clientlib/steamnetworkingsockets_udp.h / steamnetworkingsockets_udp.cpp:
  • Added m_rateLimiterRecvPackets to CConnectionTransportUDPBase.
  • Rate limiting is enforced in Received_Data() immediately following cryptographic validation (DecryptDataChunk()). This ensures unauthenticated or spoofed packets fail decryption and cannot consume tokens of legitimate peers.
• tests/test_connection.cpp:
  • Added Test_recv_rate_limit and registered in suite-quick.
  • Validates that bursts beyond the limit are dropped, and subsequent packets succeed once tokens replenish after a pause.
• Verified cleanly with ./build/bin/test_connection suite-quick.