Add custom multicast data channel

[Maryam (maryammadzadeh)]

How multicast works

• Optionally creates a second tagged Party endpoint dedicated to custom data
• Tracks only remote custom endpoints and sends one payload to all of them through a single Party SendMessage() call:
• Routes messages received by the custom endpoint away from Unreal’s normal socket/NetConnection parser:
• Collects and sends one server payload after Super::TickFlush():
• Queues received payloads and dispatches them immediately after Super::TickDispatch():
• Provides static bind-once delegates on UPlayFabNetDriver:
• Treats an endpoint not yet discovered during connection startup as transient with SE_EWOULDBLOCK. This used because SE_ENOTCONN represents a hard failure and starts the connection shut down.
• Documents enablement and binding: README.md
• The feature is opt-in through bEnableCustomDataEndpoint